Red Zone - the area inside the 20-yard line
When a team is on offense and they get to the 20-yard line closest to the end zone where they're trying to score, that is the Red Zone (from the 20 to the end zone).
You'll hear TV announcers talk about a team's "Red Zone Scoring" or the percentages of times they score (touchdown, field goal, or no score) once they've made it to the Red Zone.
Side note: We like the Tampa Bay Buccaneers. In their stadium, they have a huge pirate ship. Whenever the Bucs make it to the Red Zone, they fire the canons from the pirate ship one time. (When they score, the canons fire one time for each point scored.) It's awesome and LOUD - especially when you're in the stadium!