Cohesion is the degree to which the methods of a single class are tight together. When two methods in a class do not use a common attribute or a common method, it means that they share nothing and should probably not belong to the same class to respect the Single Responsibility Principle. In other words you can split your class into several new classes to gain modularity at class level.
The way Sonar computes the LCOM4 metric is based on Hitz & Montazeri. Here is a good introduction: http://www.aivosto.com/project/help/pm-oo-cohesion.html#LCOM4.
See the following blog post: Clean Up Design at Class Level with Sonar.