The Lowest Common Ancestor (LCA) of two nodes is the deepest node that has both of them as descendants. Think of it as the first common point when you trace paths upward in the tree. For example, if you have nodes and in a tree, their LCA is the lowest node that sits above both of them. If one node is an ancestor of the other, then that ancestor is the LCA.
This definition works for any tree structure. The root is always an ancestor of every node, but you want the lowest one that is common to both. In the next unit, I'll show you concrete examples to make this crystal clear.