What is arm pronation?

What is the definition of arm pronation? I’ve heard the phrase before but i’ve never had it explained.

I know it applies to mechanics but is it good or bad,and should I look for it when throwing with my son who is still young?

I may be wrong on explaining it but I know what it is.

Think of it as waving good bye to a ball when you throw it, all it is, is snapping your wrist and its something your son should do because it takes some of the load off his elbow it’s pretty much a natural motion so your son probably does it. Just think thumb down when he releases the ball.

The arm naturally pronates when you throw a baseball. Think palms up to palms down. That’s pronating.

Thanks for explaaining it.