First and foremost, it is only applicable to teams that play 7v7.
The line serves two purposes.
1 - To help a team start the game with possession.
Simply put - If the GK for the other team (BLUE) gets the ball (either a goal kick or picks up the ball), your team (RED) MUST move back behind the build out line. The BLUE players can go anywhere.
To get the ball in play - The GK cannot punt the ball.
They can throw / roll to their team mate. They can also put it on the ground and play with their feet (pass to a team mate).
If it is a goal kick, they must kick the ball in. If any the kick crosses the build out line in mid-air without touching another player first, this is a violation. The restart for this violation is a free kick where the ball broke the plane of the build out line.
When can the RED team pressure the ball? Your team can move back to the ball once the ball is ‘in play’ from the GK.
‘In play’ is the ball being played/put down by the GK.
Note that the GK does not have to wait for the other team to move back behind the build out line.
2 - It is an offside line.
In the 9v9 and 11v11 game, the offside line is the halfway line. At 7v7 the field has been extended for you: