It’s a short, fat worm, which mimics the chopped earthworm.
It can be used in combination with other baits when fishing with trout floats, or hooked on a jig head to let it dance on the bottom and trigger the predatory instincts of trouts; it may even be hooked with wacky style (i.e. with the hook placed in the middle of the body) with no weight, to obtain a very natural fall to the bottom.
The Fat Worms are flavored with fish taste: when the trout takes the bait, stops to taste it, allowing to strike with the right timing.