Note:This law/rule only applies to U10 and above age groups. Sometimes offsides we may choose not to apply the offsides law to the U10 age group.
Offside (sometimes known as offsides) is a law in association football which states that if a player is in an offside position when the ball is touched or played by a team-mate, the player may not become actively involved in the play. A player is in an offside position when closer to the opponent's goal line than both the ball and the second-to-last defender (which is usually the last outfield player), and also in the opponent's half of the pitch. "Offside position" is a matter of fact, whereas committing an "offside offence" occurs when a player is "actively involved" and is subject to the interpretation of the referee. Goals scored after committing an offside offence are nullified if caught by the referee.
View this Youtube video for a detailed description of the offsides law