I try to follow American football as much as I can, but having lived my whole life in Europe it's not something you get exposed to a lot. So I had to decipher the game by myself.
One particular rule that I have always had hard times figuring out is the interference. What I know is that when the QB throws a high/deep ball to a WR, the defender who's running with the WR cannot interfere with the receiver until some arbitrary point.
Seeing as the sport is a physical, contact-based one, how do you defend if you can't really tackle a player going for a touchdown or a major 1st? Could someone explain this rule in plain English (i.e. please no c/p from a rulebook)?