Kids who have trouble regulating sensory input can easily become distracted, seeking out sensation to either stimulate or calm their nervous system. Fidgeting gives them this input, usually in a way that is disruptive to both them and their classmates - wriggling, biting their nails, doodling, moving about the classroom.
A fidget toy is an object that the student can use to get this input in a less distracting way. They can help improve concentration and attention to tasks by allowing the brain to filter out the extra sensory information.