We call it a sweat shield but it is comfort feature. It is not really to keep sweat off the gun. It's a tab of leather cut as part of the holster, coming up on the body side of the holster between the slide and the wearer's side. It protects the gun and the shooter by acting as a spacer and buffer.
You'll find some people call them body shields, but no matter what they're called, they should not be oversized which interferes with the proper grip.
You should be able to take your perfect grip on the gun while it's in the holster and before you begin to lift the gun. Under stress you certainly do not want to re-grip the gun on the way to the target. Small muscles don't work well when adrenaline rules the day.
The majority of our customers request sweat shields for one other reason - they make re-holstering easier.
Cost is $15 if not a standard feature on the particular custom holster you're ordering.