Read it in context. Preferably the whole book.
At this point, Paul has said that when God told us what he wanted from us (by means of the Old Testament Law), mankind as a whole rejected it and did our own thing. Paul's basically addressing exactly what the other poster said - if God can forgive our sin, why don't we just sin more so God can forgive more? He answers that by saying that when God forgives our sin, we are supposed to be different in the future. It would be wrong to take advantage of it.
If this doesn't help, hit me on XMPP at mcivor@jabber.org and I can try again.