The multiregional hypothesis is one hypothesis about the origin of modern human beings. There are, generally speaking, two major hypotheses. The multiregional hypothesis is one of these while the “out of Africa” hypothesis is the other.
The out of Africa hypothesis holds that fully modern humans evolved only once. They evolved in Africa and then spread out from that continent. As they spread out, they replaced the populations of Homo erectus that had previously come out of Africa and colonized the various parts of Eurasia.
By contrast, the multiregional hypothesis holds that there was only one migration out of Africa. This was the migration of Homo erectus. Homo erectus migrated out of Africa and settled across Eurasia. Then, the species gradually evolved into fully modern humans but did so separately in the different regions. This is why it is called the multiregional hypothesis; it holds that fully modern humans evolved separately in multiple regions of the world.