What it measures
Every swing arc has a lowest point. Low point distance tells you how far ahead of or behind the ball that bottom is. Low point side tells you how far left or right of the impact spot it is, measured at a right angle to the target line, as seen from behind the ball looking at the target. It is a distance, and a companion number, low point height, gives the vertical gap. The club usually hits the ball or the ground before it reaches the bottom of its arc, so the low point is projected from the arc rather than seen.
As with other side-to-side numbers, a reading to the right of the target line is generally positive and to the left is negative. Left-handed golfers see the picture mirrored.
Typical numbers
There are no standard averages for low point side, and the values are usually small, because the club only travels a few inches between the ball and the bottom of the arc. As an illustration, if the low point is about 4 inches ahead of the ball and the swing direction is 3 degrees left, simple geometry puts the low point roughly two tenths of an inch left. Readings grow when the low point is farther from the ball or the swing is aimed well off line, and they vary from golfer to golfer.
What changes it
Swing direction is the main input: the bottom of the arc sits along the line the swing is aimed. Low point distance scales it. When you hit down with an iron, the low point is ahead of the ball, so low point side falls on the same side as swing direction. When you hit up on a teed driver, the low point is behind the ball, so the reading flips to the other side. The tilt of the swing plane adds a small extra shift away from the golfer, since the club is still moving down and outward toward the bottom of the arc.
What it changes
Low point side does not change the ball's flight by itself. It is a diagnostic that reflects the direction of the arc, and a low point far to one side can point to an out to in or in to out path. Outdoors you would see it in divots that point left or right of the target, so in a bay it stands in for the divot you cannot see on the mat. Read it with low point distance and attack angle to see where and how the club bottomed out, and with swing direction and club path to see why.
How to use it
Look at the pattern over many swings rather than one value, and always alongside low point distance and swing direction. If your irons show a large reading to the left together with a left path and pulled or sliced shots, the arc is working across the ball.
Bay drill: lay an alignment stick on the mat along the target line and place a headcover just outside the ball, slightly ahead of it. Hit ten 7 irons trying to miss the headcover, which discourages an outside swing, and watch low point side and swing direction move toward the target line while low point distance stays ahead of the ball.