Kneeling is when a slave rests her weight on her kneels and the tops of her feet at the same time. Further, this position can also change so the slave's body is held off the ground by the arms at the from and knees at the back, forcing the slave to crawl.
Kneeling (the Japanese "seiza" position) is considered a very polite, respectful position in many cultures. If your Master / Mistress is seated (for example watching TV or eating dinner), kneeling places you at the same height as them, without the privilege of the seat. Kneeling is also an excellent position for the slave to be able to service her Master or Mistress with oral sex, receiving punishments, or simply waiting.
At its most basic, forcing a slave to kneel removes much of their freedom of movement. Without touching the slave, the Master or Mistress is able to control them, or even humiliate if the kneeling posture also requires the slave's knees to be spread exposing themselves.