No, we're not running around in a gym, tagging other players. Capture the flag contests, or CTFs for short, are intense cybersecurity contests where participants try to capture a piece of information. To do this, they must use skills to capture a virtual flag, usually taking the form of a string, like this: flag{this_is_a_flag}. There are two common types of CTF competitions: jeopardy, and attack-defense.
CTFs are different from the hacking you may hear from the news. While hacking is commonly associated with stealing and destruction, CTFs are all about learning and community.