Since it doesn't say it anywhere on the linked page or in the headline, from Wikipedia:
Capture the Flag (CTF) in computer security is an exercise in which participants attempt to find text strings, called "flags", which are secretly hidden in purposefully-vulnerable programs or websites.
Capture the Flag (CTF) in computer security is an exercise in which participants attempt to find text strings, called "flags", which are secretly hidden in purposefully-vulnerable programs or websites.