Testing backend or frontend

I'm a beginner at coding and testing. In the school we were speaking about frontend and backend. But i sometimes do not understand what is what.

For example: when I test a game (download from net) which type of test is it? When i'm playing (testing as End user) Am i testing back or frontend?

Or, f.e. developers develop using PL/SQL, then i download a batch and it opens a software to " play with". I can change objects, add, move etc. Than what am i testing? Is it a client-server architecture?

Thanks for explanations