It all comes from the effects a particular allele’s coded protein has, when another allele’s protein is present.

If the effects of one can completely hide the effects of the other, then the hidden one is recessive, the other dominant. In ABO blood, this could be a single B allele "hiding" an O allele to produce B blood.

In codominance, neither allele’s protein can completely overwhelm the effect of the other, so both effects show up, like AB blood.

In incomplete dominance, one allele’s protein has a much weaker but detectable effect. In sickle-cell anemia, having one sickle-cell allele and one "regular" hemoglobin allele produces some symptoms, but not sickle cell disease.