A slave is a form of submission in which the submissive gives over their own will entirely to their Master. It’s a more encompassing, more entire form of submission, in which the sub doesn’t set their own limits, their boundaries are set for them by their master or mistress. They act as though ‘owned’ by their master and may or may not be masochists, but collars are quite common in this M/s play.
Like any other form of submission, there’s a spectrum in being a slave. Some want to be and are extremely micromanaged. Details like when to eat, what to eat, when to use the bathroom, and when to speak are decided by their Master. Other slaves have fewer of the outward acts but may have a slave heart. I would imagine most if not all slaves have a slave heart, and I’ll let them define that for themselves. Slaves tend to give up even more control to their Dominant partner than other types of submissives do.