What does it mean to have type "O positive" blood type? is that rare or common in people?

It means you have no surface antigens, A and B antibodies, and are positive for the Rh Factor.

Type O blood has no surface antigens and has both A and B antibodies. The positive part simply means you have the Rhesus Factor (Rh for short, named after the Rhesus monkey where the Factor was originally discovered). This is not rare at all, over 40% of people have Type O blood, the majority of which are O+. In fact I myself am O+.