A thumb hole is a round opening cut through the blade near the spine that you push against with your thumb to open the knife. It is ambidextrous by nature and adds nothing that can snag on a pocket.
Because the opener is a hole rather than a protrusion, there is nothing sticking out to catch on clothing or dig into a palm. It works identically for left and right handed users with no extra parts.
It also removes a small amount of weight from the front of the blade, which shifts balance slightly toward the hand.
A hole needs blade width to sit in, so it is rare on slim blades. It also removes steel from the blade, though from a low-stress area near the spine.
In gloves it is generally easier to use than a small thumb stud, because there is more surface to catch.