Blocking is the most underappreciated facet of the game. A good running game can be attributed to a good offensive line. Same for a good passing game. The line must be able to hold blocks long enough for receivers to get open and runners to move down field.
Blocking has strict form. You must block by pushing off the other player, not by holding on to them. The idea is to obstruct another player using your body. You are also forbidden from grabbing onto the jersey or any piece of equipment. When players line up at the line of scrimmage (pictured), they can see whom they are going to be blocking. It is a highly physical position.
Usually it's the offensive line that does most of the blocking, however other players may be asked to block on certain plays to create room for players attempting to run the ball or to buy time for the quarterback.