Thank you for your reply.

Yes I am an undergraduate student so I am mostly asking about basic things

We use quite advanced book ''Topology, geometry and physics'' of Nakahara. His book is quite difficult because it is directed to graduated students and requires very much mathematical preliminaries (which undergraduate students quite ofthe dont have)

I have an idea what the adjoint representation might be but I have problems with some physical interpretation of it which is most important in this case.

Writting just a definition and some formulae does not help with understanding the whole concept behind it at all. It just makes it boring for the reader. It is always very helpful to try to interpret physically and give examples.