The teams swap between being on offense and defense, offense you try to score, defense you try to stop them from scoring. Essentially you have to get the ball carried by someone past 10 yards without dropping it, that's call a first down, you get technically 4 chances to get there but most teams choose to punt the ball at the 4th chance because if they fail the enemy team gets the ball at wherever they are, so it's usually better just to kick it away. The goal is to make it to the endzone where if you have a guy in there who is carrying the ball it is a touchdown and the team gets 6 points, they can then choose to either kick a field goal for one point which is easer, or go for two points where they basically have to get another touchdown at the 5 yard line. If a player on the defensive team catches the ball (an interception), or an offensive player drops the ball that they had control over and a defensive player picks it up (a fumble), that team now has the ball and it is now offensive for them. Who bunch of penalty shit too but that's the general stuff that happens.
Sorry if this wasn't explained well, I didn't realize hard it would be to try to explain to someone who doesn't know anything. If you any questions I may be able to answer them. I'm not much of a football guy but I've watched it from time to time