Capture The Flags or CTFs as they're called are a kind of computer security competition.
Teams of competitors are pitted against each other in a hacking competition. Teams earn points by hacking competition challenges (a Jeopardy CTF) or by hacking each others infrastructure (an Attack & Defense CTF).
Once a team solves a challenge, they receive a flag that they can submit for points. The team with the most points at the end of the competition wins.
Hundreds of CTFs happen every year and that number is only growing. They've been used since at least 1996 by hackers looking to test each others skill. CTFs have excellent properties promoting problem solving and team building. Not only that but CTFs can expose participants to completely foreign topics thtat they might never encounter in their day to day activities.