Where can we find easily documentations about fonctions/methods provided by libraries ?

Hi everybody, I'd like to code my own plugins. I did the tutorials from the web site but if I look at the examples, in the .cpp files, many different methods are used to describe the plugin. The problem is that I don't know what these methods do. Moreover, I'd like to use all the functionalities provided by the libraries to be more efficient. So, my question is : Where can we find easily documentations about functions/methods provided by libraries ? Maybe on the Gazebo website ? Or directly in the library folder ? Thank you in advance Ronan