Sack the quarterback in his own end zone. In football, each team has its own goal to defend. This is the "end zone" at the end of the field. When a team is beginning an offensive drive down the field, they sometimes begin close to or just in front of their own end zone. If the defensive team can tackle---or sack---the quarterback in his own end zone, the defensive team will score two points. This is called a "safety."
Tackle any other offensive player, such as a running back, who is carrying the ball in his team's end zone. This also results in a safety.
Run a "two-point conversion" after scoring a touchdown. Once a team gets the ball into their opponent's end zone, they score a touchdown. This is worth six points. Most teams will try to kick an extra point, a field goal from just outside the end zone that is worth one point. To earn two points, however, the offensive team gets one play to run or pass the ball into the end zone by starting from the opponent's 2-yard line, thus earning a "two-point conversion."