Help in understanding Trees & Graphs

I’m really unable to grasp the concept of Trees & Graphs like I am unable to figure out any approach. I also try to understand and see the solution of others and I save the problem to come back later and solve it, but I am unable to do so. So if there are any resources out there that helped you all, please let me know.

I think this will help graph theory
this man created some very good tutorials for graph theory and many other concepts of cp