Unicellular organisms have a simple cellular structure. They reproduce by simple methods of reproduction like binary fission, multiple fission, budding etc.
A multi-cellular organism have a complex cellular structure, have specialised functions. So reproducing by simple methods does not work. So for reproduction, they develop reproductive organs like flowers in plants, Ovaries and testis in an animal.
These organs produce gametes. Gametes on fertilisation and growth result into new individual organisms.