It really isn't a difficult concept to understand.
Pro-choice, means that we believe it is the individual woman's right to choose what happens to her own body. We believe that it is the woman's choice whether she wishes to carry a pregnancy to term, or not. It is not about being for or against abortion, it is about supporting the individuals right to decide for herself, it is supporting the autonomy of the individual woman.
A person can be pro-choice, without being pro-abortion.
If I personally don't agree with abortion, that is my choice. What I don't have the right to do, or anyone else has the right to do, is make that choice for another.