I recommend watching the following videos for learning about graphs. These are really well made videos and he covers a lot in these short videos, so if you feel overwhelmed, use the rewind and pause buttons.
What are graphs? What are different types of graphs (directed and undirected, trees and DAGs, etc)? And examples of using them to model web links, flights between cities, and other things.
How to store (or represent) a graph in code? i.e. adjacency list and adjacency matrix. When do you use one vs the other?