To make someone feel afraid or frightened.
Scaring someone is usually not a nice thing to do.
Examples:
The loud noise will scare the cat.
I don't want to scare you, but there's a spider on your shoulder.
The dark can scare little kids.
I will scare my brother with a funny mask.
The monster under the bed can scare you at night.
The big dog scared the little dog away.