What is the meaning of cells of multicellular organisms are speclized?

Specialised cells mean they have one specific job, for example cells in the alveoli (in the lungs) would be specialised to transfer oxygen into the blood. An unspecialised cell would be an embryonic stem cell which can become any cell to suit the needs of the organism, which is how a small cluster of cells in a zygote becomes a full human with all the organs and such.