What does believe in one god mean?

There are a number of different possible religious beliefs about God or gods. Atheists believe that there is no god. Jews, Christians, and Muslims believe that there is one and only one God. Hindus and a number of other religions believe that there are many gods. (In Hinduism there are three main gods, Brahma, Vishnu, and Shiva, but there are lots more.) So if you believe that there is one and only one God, then you believe in one god.