AVL Tree

Question 1

What does this prevent?

Answer 1

Prevents the binary tree from ending up like a straight line/ending up like a linked list again where every element ends up on one side of the initial node.

Question 2

What is the concept?

Answer 2

The height on the left and right of any given node has a height difference of no more than 1. This goes for every node on the tree, not just the starting node.