Humblers are hard to describe without showing you an image of one in use. But unless you're actually in to it an image of one in use might be a bit too much for the eyeballs, so I'll try my best to just explain.
A humbler is usually fairly heavy, made of wood or metal and in like a shallow wave shape and in two pieces that are bolted together. The two ends of the 'wave' are to rest on the back of a mans thighs (at the base of his butt) when in a kneeling or all fours position. Meanwhile the middle of the humbler is clamped around the base of the scrotum, so that the balls are trapped round the back. If the dude wearing the humbler tries to straighten his legs, the humbler will PULL ON HIS SACK causing SIGNIFICANT DISCOMFORT.
I believe it's name stems from it's usage of keeping wearers in a 'humbling' kneeling position.