Not sure what answer you're looking for. The main page of the main site already explained what it is.
Basically it is a fork of bitcoin core which won't change the network rules (e.g XT and classic). It have more features but the new features are not fully tested and may cause problems.
There's an article about
here if you're interested.
well , that's the issue ... I thought developers are against XT and forks of Bitcoin Core since theymos and other people consider it as an alternative coin, If it's the case then how come It's listed on Bitcoin Core website
XT, classic, unlimited, etc. make changes the network rules and that's why theymos and others are against them. Knots does not, it is using the same rules as core.
and If the fork happened in December 2011 , how come It just poped up in Bitcoin Core (after five years)
I have no idea. Tho I don't think it matters.