Soccer is a contact sport played with eleven players on each team as shown on bundesliga. As in many sports the team with the most points at the end of time wins.
One team starts the game by the kickoff at the center of the field. The ball is usually passed forward just a tad for that start.
Then it is game on. Teams try to score. The players kick the ball and they can also use their body to touch the ball but not their arms or hands. The goalies can use their hands when inside the big box.
Teams try to strategically use or avoid penalties. Penalties stop play for out of bounds, hand balls, roughing, slide tackles, corner kicks, penalty kicks, and things like offsides.
So what is offsides? The offense cannot line up ahead of the defense unless they already have the ball. If a pass is or a shot is attempted while a player is offsides a penalty whistle blows giving the defense a free kick in their defensive zone.
When a goal is scored sometimes it is in question. If that is the case it can be reviewed to see if a penalty negates the goal. If it turns out that a rule was broke the goal does not count. Then the subsequent penalty would play out resulting in the correct free kick or throw in.
If a player commits a bad penalty or two near bad penalties they can be removed from the game. That is called a red card. If in the defensive big box a penalty occurs it also results in a penalty kick. That is a point blank shot by anyone in the game. When a player is removed for a red card they cannot return. But unlike a substitution no one replaced them; the team just played one player down.