I had trouble understanding recursion in the section on traversing binary trees in Part 2 of the Data Structures & Algorithms Course. So I did some extra homework and found a couple resources that improved my understanding.
If you are also uncomfortable with recursion, I hope this helps:

Recursion for Beginners: A Beginner’s Guide to Recursion
Al Sweigart explains the role of the call stack in recursion (“to understand recursion, you must first understand stacks”), plus when to use recursion vs loops 
Recursion is not hard: a stepbystep walkthrough of this useful programming technique
An excellent written explanation of the use of stacks in recursion