Pedal kickback on an mtb, what is it?
On a full-suspension bike, the rear chain- and seat-stays are attached using pivot points. However, what attaches to the dropouts on the chainstay? The wheels. The chain's route is constantly changing as the suspension travel changes. If the suspension is fully compressed, the distance from the wheel to the pedal decreases. If the suspension is fully de-compressed, the distance increases. This essentially changes the amount of chain links between each side. If the increase is sudden, which the back wheel moving at a constant rate, it'll either increase the free-hub/wheel speed or will 'kick' the pedals back. It's an un-nerving feeling, but it essentially means that you're, in a sense, pedaling without actually pedaling as your force downwards is kind of amplified as though your gearing has temporarily changed. However, when the frame pivots re-extend, they usually over-extend before settling, undoing all that awesome work that the pedal-kickback has just done for you.

This is a painfully hard topic to explain in words, and is usually only suitable to be taught using illustrations and videos, so here's a photo I've found from the internet:

This image is from: "https://polebicycles.com/wp-content/uploads/2015/06/Pedal-Kickback.jpg"

It's a bit math-y, but hopefully it'll help you understand how it works in a sense.