Pushback is the stun caused by taking an attack that doesn’t blow you away or knock you down. This state involves having this stun on the ground.
This is also known as “hitstun”, or “hit stop”, and is generally thought of as being accompanied by “knockback”, though the nuances of each term apply to this same phenomenon in different ways (see respective articles for details).
A lot of combos are made by making specific attacks hit during this period.
Contrast this with the floating state, where combos that attack it are called “juggles”.