To stop the offense from advancing the ball, the defense must tackle the player with the ball by knocking or pulling them down (as every inch counts). As such, defensive players must use some form of physical contact to bring the ball-carrier to the ground, within certain rules and guidelines.
Tacklers cannot kick or punch the runner. They also cannot grab the face mask of the runner’s helmet or lead into a tackle with their own helmet. Despite these and other rules regarding unnecessary roughness, most other forms of tackling are legal. Blockers and defenders trying to evade them also have wide leeway in trying to force their opponents out of the way.
To compensate for this, players must wear special protective equipment, such as a padded plastic helmet, shoulder pads, hip pads and knee pads. These protective pads were introduced decades ago to reduce the risk of head and neck injuries and have improved ever since to help minimize injury to players. The continued development of the safety equipment over the decades has resulted in increasing levels of contact in the game with players making contact at high speed.