Offsides is the most confusing rule in soccer. In a nutshell, the offside rule prevents cherry-picking. If an offensive player were to just hang out by the goal hoping for a big kick that clears the other players, he or she would have a clear advantage over the goalie. The offside rule prevents this. This rule is not introduced until 3rd grade.
In this image, the blue player nearest the goal is in an offside position because there is only one defender (the goalkeeper) between him and the goal and he doesn't have the ball. A referee will not make a call simply because a player is in an offside position, however. A call is only made when the ball is passed and the player attempts to play it or interferes in play. In other words, if an offensive player is just slow to move back and is not involved in the play, there is no call.
Additionally, a player may not be in an offside position on his own half of the field. In the case of the build-out line at the 3rd/4th grade level, a player may not be in an offside position unless they are between the build-out line and the goal. Using the build-out line instead of the center line for this level means fewer offside calls and an easier transition.
To be even more particular, an offside call is determined when the ball is kicked, not when it reaches the player. A fast player may be in an onside position when the ball is kicked and reach the ball beyond the defenders without ever being offside.
Here is a pretty detailed explanation with pictures: https://www.washingtonpost.com/sports/interactive/2022/offside-world-cup/.
If players (or parents) don't fully grasp the offside rule, it's ok. Coaches and referees will remind players throughout the games.