In Unlabeled tree's only structure matters, what's value in node that does not matter.
1. o 2. o 3. o 4. o 5. o
\ \ / \ / /
o o o o o o
/ \ / \
o o o o
With three unlabeled node, there are 5 trees. If they were labeled, then each strucure can be filled with 3! ways.