What does a left shift mean on a cbc blood test?

Usually infection: Left shift means slightly pre-mature leukocytes in circulation which is usually due to infection.

More immature cells: Left shift means that the white blood cells are more immature. Depending on the degree of immaturity (blasts, metamyleocytes, promyelocytes) would determine what is wrong. Infections are a common cause of left shift, but other hematologic disorders are too: these would include bone marrow diseases, lymphoma and leukemia, among others.