What is a Backdoor

How would I actually protect my server against a backdoor? If I'm putting addons in then that's definitely something to avoid.

Backdoors are inserted into code by people who want to troll you or steal your files. On Garry's Mod servers, people can insert code into leaked addons which tell whoever made the backdoor the IP of your server which will allow them to come onto your server and do a hidden command usually giving them superadmin. Other backdoors do things such as give the backdoor maker access to your server files.

A backdoor is some malicious code that someone put into a leaked addon that can be used against you and your server to take it down/download your data or just in general fuck it up.