When the ball is being served:
If the line judge in charge of the line which the server is serving from waves his/her flag in the air to gain the main referee’s attention, it is called a foot fault. This occurs when the server’s foot crosses over the back line and lands within the court before the ball is contacted. This includes if any part of the player’s foot is contacting the line.
This rule makes jump serves possible. If you contact the ball before your foot hits the ground beyond the line, it is completely legal, so you can jump over the line, hit the ball, then land.
When the ball hits the ground:
If they are pointing in a downwards direction, it means that they see the ball as landing within the bounds of the court, on their line. For example, they could be looking down the long side of the court, and could be looking from one corner to the other, and the ball lands on the court side of the line, but outside of the far line which is not that judge’s call.
If they are pointing in an upwards direction, it means that the ball has landed outside of the bounds of the court, which includes the lines. So if any part of the ball lands on any part of the line, it is considered in.
If they are holding their hand and flag in a “T” position, or run one hand over the fingertips of the other, while holding them up in the air, it means that a defending player has contacted the ball while it was in flight. This is important, because if a defending player contacts the ball, and it lands out of bounds, it is the opposing team’s point.