In a nutshell
ALOHA lets stations on a shared channel transmit whenever they have data, with no checking first. If two transmissions overlap they collide and both are lost, so each sender waits a random time and tries again. Pure ALOHA allows starts at any instant and tops out around 18% channel usage; Slotted ALOHA forces every transmission to begin on a fixed time boundary, which removes partial overlaps and roughly doubles that to about 37%.