The end zone is an area on each side of the field where touchdowns are made. Each team defends an end zone and tries to stop their opponents from gaining yards down the field and getting in their end zone.
The end zone is composed of three types of lines:
Scoring a touchdown in football gives your team six points. To score a touchdown, the player must carry or catch the ball in the opposing team's end zone and remain inbounds with control of the ball. The ball carrier must also break the plane of the goal line, meaning he and the ball must go over the goal line.
A safety is worth two points for the opposing team and happens when the offensive team's ball-carrier makes a mistake in their end zone. For example, if the offensive team gets tackled, runs out-of-bounds, or commits a penalty, then the defensive team is awarded two points. This is the only way the defensive team can gain points without being in possession of the ball.
The goal posts sit at the back of the end zone. A team can kick a field goal from anywhere on the field and on any down. Field goals are worth three points and are scored when a kicker successfully kicks the ball above the crossbar and in between the uprights of the goal post.
The goal line marks the front of the end zone. You can imagine the goal line as a 3D plane. The ball must break this imaginary barrier for a touchdown to be scored. This is at the discretion of the referees. Sometimes instant replay is needed to justify a touchdown. Coaches can use a challenge flag just for this purpose.