Just to give the actual answer, CTF in this context means a computer security competition. Generally the way they work, is you get some programs, and you have to hack them to get some string called the flag (e.g. maybe the server has a root owned file called flag, so you have to get root somehow to read the file). Team with the most flags at the end wins.

In this context, CTF is almost exclusively referred to by the initialism, i think to help distinguish from other uses of the term.