Under 12’s and up play with Offside. Each team must supply one Assistant Referee (Linesman/Lineswoman) from the Parent group for home and away games.
The number one job of the Assistant Referee is wave your flag when a player is offside to alert the Referee.
Glad you asked, have a look at this very basic intro to Offside.
Short Videos showing offside to help explain the rule.
Should you really want to get into the nitty gritty there is the FA website on Rule 11
Best practice when new to the role is in line with the last Defender and don't go past the centre line to the other end of the pitch.
This last Video is from US Soccer and it contains advice for those starting including positioning.
We don't expect our parents to run as much as these Assistant Refs do.