To debrief means to question someone to get information about something that happened.
Debriefing is usually done after something important or serious happens, like a big event or a problem.
Examples:
The police will debrief the witness about what they saw.
After the game, the coach will debrief the players about how they played.
She debriefed the students about their field trip.
I will debrief my little brother about what happened at the party.
The aliens debriefed the earthlings about their strange customs.
The teacher debriefed the class after the big test.