"Trimming" something means cutting it just a little to make it look nice. When you "trim" someone's hair, you just cut it a little bit.
You can talk about what part of a person's hair is being cut:
She trimmed the bottom.
Can you trim the sides and leave it long in the back?
Aside from hair, you can also "trim":
- a beard or mustache
- a hedge (a wall of bushes)
- fat from a piece of meat