It means you are sexually attracted to members of both sexes. The prefix "bi" means 2. So bisexual means 2 sexes.
Bisexuality is described in many different ways depending on individual experience. Generally, it means you are attracted in some way to both sexes. This does not have be and equal attraction and does not have to occur at the same time. Many people prefer certain aspects of one sex over the other, but still appreciate and are attracted to both. Some people are attracted to different sexes at different times in their lives. Some people realize they are bisexual when they are very young, others when they are much older.
Overall, it is a very subjective and unique experience. But, generally speaking, in the barest of terms, being bisexual means you are attracted to both sexes.