Well, if you want to participate in the discussions, you need to learn the rules. Football (I'm not calling it Soccer) is like a religion in Europe and South America. The rules are fairly simple and easy to learn. I personally find the American Football a lot more complicated.
The rule you are referring is probably this one:
http://en.wikipedia.org/wiki/Offside_(association_football)
The basic idea is that you can't have someone from the attacking team just standing behind the defending team's defense and waiting to receive the ball and score. The game can't function properly without this rule, since nobody will be trying to outrun the opponents anymore.