To cause someone to be surprised or frightened; to startle or give someone a shock.
子どもたちはお化け屋敷で友達を驚かせて大笑いしていた。
The children scared their friends at the haunted house and were laughing loudly.
そんな急に出てこられると赤ちゃんを驚かせるよ。
If you come out so suddenly you'll startle the baby.
彼女の告白はクラスのみんなを驚かせた。
Her confession surprised everyone in the class.