VAC stands for Valve Anti Cheat and it is a system put in place by Valve to protect games from the use of any unfair third-party modifications.
How does it work?
If a user connects to a VAC-Secured server from a computer with identifiable cheats installed, the VAC system will ban the user from playing that game on VAC-Secured servers in the future.
The VAC system reliably detects cheats using their cheat signatures. Any third-party modifications to a game designed to give one player an advantage over another is classified as a cheat or hack and will trigger a VAC ban. This includes modifications to a game's core executable files and dynamic link libraries.
For some additional information regarding the time of the ban, VAC bans are delayed and the action that triggered the ban would have happened well before the ban itself came.
What do I do now?
VAC bans are not controlled by the game developer but by Valve itself so you will have to contact their support team here. For further information regarding VAC, please refer to the following steam article here.