o que significa o phrasal verbo to tell someone off

While I was watching TV the other day I saw an interratial couple discussing possible situations they might have to confront from that point on now that they're together. The black woman said it's the first time she's been in a relationship like that but the white man claims to have dated black girls in the past and brings up an experience he had with an ex while holding hands on the streets.

He didn't give details but basically a random guy shouted insults to them out of nowhere and for no reason other than prejudice. He said his girlfriend got really mad and went up to him to tell him off. When you tell someone off, you "castigate, rebuke, or reprimand one in a severe or merciless manner". A noun or pronoun can be used between "tell" and "off." Parents tell off their children very often for their bad behavior for example.