The offsides rule

Offsides is a rule that prevents players from sitting near the other team's goal to receive a pass and score a goal while not having a defender in position to stop the shot.
It only applies in the opponent's half of the field.
There must be 2 defensive players between the offensive player and their goal when a pass from further upfield from the goal is kicked to be received by the offensive player. This is usually the goalie and a defender but if the goalie is upfield, there must be two defenders closer to the goal than the player being passed to.
Once the ball is kicked by a teammate, if they started in an onside position, they can run to the ball and play it even if they beat everyone to the ball and they are then closer to the goal than any of the defenders.
The ball counts as a defender. Thus the reason for taking the ball down near the goal line and passing it back to the middle away from the goalie.
Sometimes a line referee will signal an offside position without seeing if that player is involved in the play. The assistant referee is only supposed to signal the offsides if the player that was in an offside position is involved in the play. If the player that is offsides makes no attempt to play the ball, the offsides should not be called, but it is sometimes.
Being exactly even with the defender is not offsides. However, an arm of head or foot is enough to allow the assistant referee to call an offsides.