Oral sex is when the slave uses her mouth to lick or suck her Master's penis, or when she licks her Mistress' vagina.
In BDSM play, the Master or Mistress may wish to go further and actually force their slave to perform the task. This could be done by threats ("I'll spank you!"), by holding the slave's head, or by engaging in oral sex when the slave is bound and unable to resist.
Just like normal oral sex, forced oral is extremely popular in BDSM.