It is th cyclomatic complexity, also known as McCabe metric. It all comes down to simply counting 'if', 'for', 'while' statements etc. in a method. Whenever the control flow of a method splits, the Cyclomatic counter gets incremented by one.
Each method has a minimum value of 1 per default except accessors which are not considered as method and so don't increment complexity. For each of the following Java keywords/statements this value gets incremented by one:

Note that else, default, and finally don't increment the CCN value any further. On the other hand, a simple method with a switch statement and a huge block of case statements can have a surprisingly high CCN value (still it has the same value when converting a switch block to an equivalent sequence of if statements).
For instance the following method has a complexity of 5

Afferent couplings

ca

A class afferent couplings is a measure of how many other classes use the specific class.

Depth in Tree

dit

The depth of inheritance tree (DIT) metric provides for each class a measure of the inheritance levels from the object hierarchy top.

Efferent couplings

ce

A class efferent couplings is a measure of how many different classes are used by the specific class.

File dependencies to cut

package_tangles

Number of file dependencies to cut in order to remove all cycles between packages.

LCOM4

lcom4

Lack of cohesion of methods. See LCOM4 documentation page.

Number of children

noc

The number of children of a class metric measures the number of direct and indirect descendants of this class.

Package cycles

package_cycles

Minimal number of package cycles detected to be able to identify all undesired dependencies.

Package dependencies to cut

package_feedback_edges

Number of package dependencies to cut in order to remove all cycles between packages.

Package tangle index

package_tangle_index

Gives the level of tangle of the packages, best value 0% meaning that there is no cycles and worst value 100% meaning that packages are really tangled. The index is calculated using: 2 * (package_tangles / package_edges_weight) * 100.

Response for class

rfc

See RFC documentation page.

Package edges weight

package_edges_weight

Total number of file dependencies between packages.

File cycles

file_cycles

Minimal number of file cycles detected inside a package to be able to identify all undesired dependencies.

Suspect file dependencies

file_feedback_edges

File dependencies to cut in order to remove cycles between files inside a package. Warning : cycles are not always bad between files inside a package.

File tangle

file_tangles

file_tangles = file_feedback_edges.

File edges weight

file_edges_weight

Total number of file dependencies inside a package.

File tangle index

file_tangle_index

2 * (file_tangles / file_edges_weight) * 100.

Number of javadoc, multi-comment and single-comment lines. Empty comment lines like, header file comments (mainly used to define the license) and commented-out lines of code are not included.

Density of comment lines = Number of Comment lines / (Number of Lines of code + Number of Comment lines) * 100

With such a formula:

• 50% means that the number of lines of code equals the number of comment lines
• 100% means that the file only contains comment lines

Duplicated blocks

duplicated_blocks

Number of duplicated blocks of lines.

Density of duplication = duplicated_lines / lines * 100

Active reviews

Number of active reviews (status not closed).

New violations

new_violations

Number of new violations.

New xxxxx violations

new_xxxxx_violations

Number of new violations with severity xxxxx, xxxxx being Blocker, Critical, Major, Minor or Info.

Violations

violations

Number of violations.

xxxxx violations

xxxxx_violations

Number of violations with severity xxxxx, xxxxx being Blocker, Critical, Major, Minor or Info.

Weighted violations

weighted_violations

Sum of the violations weighted by the coefficient associated to each severity (Sum(xxxxx_violations * xxxxx_weight)).
To set the weight of each severity, log in as an administrator, go to Settings > Configuration > General Settings > General and set the Rules weight property.

Rules compliance

violations_density

Rules compliance index (RCI) = 100 - (weighted_violations / nloc * 100)

Number of getter and setter methods used to get (reading) or set (writing) a class property.

Number of methods.
Number of paragraphs for Cobol. 

Notes for Java:

• Accessors are considered as methods if the sonar.squid analyse.property.accessors is set to false
• Constructors are considered as methods

TODO

Java:

Number of statements as defined in the Java Language Specification but without block definitions. Statements counter gets incremented by one each time a following keyword is encountered: if, else, while, do, for, switch, break, continue, return, throw, synchronized, catch, finally.

Statements counter is not incremented by a class, method, field, annotation definition, package declaration and import declaration.

On each line of code containing some boolean expressions, the branch coverage simply answers the following question: 'Has each boolean expression been evaluated both to true and false?'. This is the density of possible branches in flow control structures that have been followed during unit tests execution.

It is a mix of Line coverage and Branch coverage. Its goal is to provide an even more accurate answer to the following question: 'How much of the source code has been covered by the unit tests?".

On a given line of code, Line coverage simply answers the following question: 'Has this line of code been executed during the execution of the unit tests?'. It is the density of covered lines by unit tests: