Seiji's Quirk grants him the ability to manipulate raw human flesh by touching it. Seiji can only manipulate the flesh of others by kneading and rolling them into a ball. He can manipulate his own flesh to a greater degree. Meatball allows Seiji to eject his flesh from his own body and control it in mid-air so it can be used as a weapon or projectile.
Seiji mainly uses his Quirk by manipulating his own flesh to create meat projectiles for offense or meat shields for defense. Seiji's meat attacks can be countered if they're destroyed with other projectiles, but he can simply reform them. His meat shields were capable of withstanding one of Katsuki's Armor Piercing Shots. If he's able to touch anyone with the aforementioned flesh weapons, Seiji can quickly transform that person into a ball of flesh and completely immobilize them.
People who have been turned into balls of flesh still maintain all their senses. Seiji must maintain a certain level of health and focus in order to maintain these flesh balls. If he's heavily injured then he will lose his grip on the flesh and his victims will return to normal.