The Violinist Argument (argument from bodily autonomy) sets up an analogy for why abortion should be acceptable. Imagine that there is a world famous violinist - a skilled and important member of society. Unfortunately, the violinist falls ill and the only way to save him is for him to be hooked up to your body for nine months. You are his life support.
We all recognize that if such a thing would be imposed upon you, it would be unjust. A person, no matter how valuable or famous, does not have the right to use your body if you don't want them to. The argument extrapolates from this conclusion that abortion, then, is legitimate, as it is merely the refusal to allow another person to use one's body for their sustenance (see the full argument here).