What it measures
Swing radius describes the curve of the clubhead's path in the last stretch before it reaches the ball. It is the radius of the circle that best fits that piece of the arc, given as a length. It is not a measure of how wide your backswing is at the top; it only looks at the bottom of the downswing.
There is no left or right sign. It reads the same way for right and left-handed golfers, and a bigger number simply means a wider, gentler curve through the hitting zone.
Typical numbers
There are no standard tour or amateur averages for swing radius, so the most useful comparison is with your own swings. As a rough sense of scale, the radius is related to the distance from your lead shoulder to the clubhead, so a driver reads larger than a 7 iron, which reads larger than a wedge, and taller golfers read larger than shorter ones. Because the body keeps moving toward the target through impact, the arc near the ball can be wider than that static length would suggest. Short pitches and chips with softer arms usually read noticeably smaller than full swings.
What changes it
Club length is the main factor, and choking down on the grip shortens the radius. A lead arm that stays extended keeps it wide, while arms that fold or pull in before impact shrink it. How and when the wrists unhinge changes the shape of the arc in this window, and a pivot that moves the hands forward toward the target flattens the bottom of the arc, which shows as a larger radius.
What it changes
Speed: at the same rate of rotation, a clubhead on a wider circle travels faster, which is why a longer club, with its wider radius, has the potential for more speed. That shows up as club speed and ball speed.
Strike: the radius also sets how quickly the club's angle changes near the ball. On a wide arc the attack angle changes slowly as the low point moves, so a strike an inch early or late still produces similar contact. On a tight arc the same timing error changes the attack angle and contact more, which is one reason a consistent radius helps chips and pitches come off cleanly.
How to use it
Use swing radius mainly as a consistency check, especially in the short game, and compare it with club speed when working on power. Do not try to force width by stiffening the arms, which usually costs speed and rhythm.
Bay drill: pick one wedge and a 30 yard target, keep the same grip length and arm structure, and hit ten pitches. Note the swing radius, carry and strike on each, then try to narrow the spread of the radius. When it stays steady, check whether your carry distances and strike quality tighten up too.