A blocker is a player on the special teams similar to a lineman on offense or defense. Blockers specialize in blocking opposing players from breaking through the line to tackle the kicker. Blockers are defensive players who act after the ball is snapped.
Blocking in football requires blockers to run into each other with force and prevent other players from getting forward progress. Blockers will use their hands to push opposing players in the opposite direction. This is one of the main reasons that players wear helmets, because collisions are a direct result of blocking, so the helmets help to alleviate some of the effects that can come from colliding.
Tackling is the act of grabbing and pushing another player to the ground. Tackling the ball carrier is an important job of the blocker. This might be the most important role of being a blocker. Nothing will stop the game until the ball carrier is tackled or loses the ball.
Holding is a penalty in football that is given to a player that holds on to an opposing player preventing their movement. This is a bit different than blocking. Holding results in a loss of 10 yards. Blockers must be careful not to grab onto the jersey of an opposing player, or they risk getting called for holding.