What is MTU on a Linksys Router and what does it affect? I want to change the MTU so I can use XBOX LIVE?

On my xbox 360, it say the MTU must be, at a minimum, 1386. My router's MTU setting is on auto and it is at 1500. In our house, we have 4 laptops and 3 desktops, so i was wondering if having all of the computers at the same time could be maxing out the MTU setting of 1500. If I were to change the MTU, would anything else get messed up or any other settings changed?