[security] mark

I have noticed that some packages on the lists of packages (e.g. 
https://packages.debian.org/stable/)  are marked as [contrib], 
[non-free] or [security]. While the first two of them are explained in 
Introductory Notes (https://www.debian.org/distrib/packages.en.html), 
the [security] one is not.

I would like to ask you to publish the explanation in the mentioned 
Introductory Notes what does the [security] mark mean.

Thank you in advance.

Sincerely yours,
Jacek Uryga