Most scholars believe that some members of the Corinthian church who did not believe in the resurrection also practiced some sort of baptism for the dead. Paul does not condone such a practice, but points out the lack of logic in their actions, since they believed two contradictory things:
1. The dead in Christ will not raise.
2. The dead in Christ who were not baptized in time need someone to be baptized on their behalf.
Still, as Jim said, no one can know what this verse means for certain. Either way, the verse does not present any theological problems and knowing what it means is not likely to influence your life in any way. Look into it more if you are really interested, but do not stress over it too much. Baptism for the dead is not and has never been a Church practice, so the verse certainly is not teaching that, because we would have at least seen it mentioned in the writings of the early church fathers.