This page gives a good visual overview of numerous Git operations. If you understand Git in principle, but are unsure about the exact meaning of certain commands, then this may be useful; for each operation covered it gives a description and a block diagram of how it changes the state of the repository.
If you’re completely in the dark, or if you don’t understand what a DAG is then you might be better off with some of the introductory documentation.
Bonus: The images are all drawn using TeX and a package I’d never heard of called TikZ. The examples page is mindblowing.