Submission is the act of complying to the will or authority of another persona and what puts the S in D/s.
Submission is not about giving up on your free will. It is, however, about letting go of control and allowing someone else to take over for a while. That can mean anything from letting a partner call the shots in the bedroom to living an entire lifestyle of servitude.
A submissive person, or a sub, will receive pleasure from being dominated and will find being in control an exhausting experience. For a sub, having their partner take the reins puts them in a relaxing, comfortable place.