Does item quality decrease over time

Team Collision determines whether you can walk through your team-mates or not.

If team collision is on, then your team-mates will act the same way as an enemy when you come into contact with them, namely, they will be able to block doors and the like with their body.

If team collision is off, then you will be able to freely walk through your team-mates.

This is a mostly anti griefing option, so that people can't purposely block doors to prevent their team-mates from getting out of spawn. It is also a tactical concern, as you can't stuff a bunch of players in one position if team collision is on.

Keep in mind that enemies will always have collision on.

It's a custom Valve's variation of the Elo Ranking System and no official documentation has been released.