The sample is then centrifuged at 2500 rpm for 10 minutes, causing the cells to form a whitish clump (pellet) at the bottom of the tube and allowing you to pour off the supernatant¹ without losing your sample.

¹the usually clear liquid overlying material deposited by settling, precipitation, or centrifugation (http://www.merriamwebster.com)

Centrifugation is a laboratory process to separate materials by density (the ratio of mass to volume). If you throw a rock in a pool of water, gravity will cause the rock (which has greater density than the water) to descend to the bottom of the pool. Centrifugation speeds up this process by applying a centripetal force to the materials in the tube. When the tube is spun, this force causes materials to move toward the bottom of the tube in order of density. In this case, cheek epithelial cells are denser than the saline solution and the cells move to the bottom of the tube to form a pellet.