Ok so I signed up for this forum and it said: Sorry you need to enable cookies to login

What does cookie mean?

a cookie is a file that stores information on your computer. it can be used just for tracking when you're logged in somewhere like this forum, or it could be used to store your password if you chose to do so, so that you could automatically log in.

so for the purposes of this forum, unless you have automatic login enabled, it just serves to allow you to be logged in.