"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