To get a haircut; to have one's hair cut (i.e., have someone cut your hair).
明日、髪を切る予約を入れた。
I made an appointment to get my hair cut tomorrow.
夏だから短く髪を切ったほうが涼しい。
Since it's summer, it's cooler if you cut your hair short.
卒業式の前に髪を切りに行くつもりだ。
I plan to go get my hair cut before the graduation ceremony.
To cut someone else's hair; to give a haircut (i.e., the act of a hairdresser/barber cutting hair).
彼は友達の髪を切るのが上手だ。
He's good at cutting his friends' hair.
休みの日に父が子供の髪を切ってあげた。
On his day off, the father cut his child's hair for him.
美容師が丁寧に髪を切ってスタイリングした。
The stylist carefully cut the hair and styled it.