Hi Sytraxz
A walk-on is where players with their own kit play but not a tournament, sort of like a typical rental customer day but with your own marker etc. Many novice teams in the past have been formed by a bunch of guys who kept on meeting up at the same walk-ons.
Most sites won't let you walk on & mix it up with the rental customers 'cause it can put the rentals off seeing you in all your glory - even if your shiney new spider is actually not as good as the site markers they will assume it is (yes, there are sites that use Impulses or even Angels as rental markers). There were also a few sad guys in the past that used to go "punter hunting", seeming to think it was really big for them to take out a dozen rental guys in one game - it's not much fun on your first ever game to be mugged with 15 shots in the back of your head & tends to put people off coming back.
If you find a decent walk-on site make friends with the owner, help them maintain their fields, buy them beer (or chocolates & flowers).