What does "incompletely dominant" mean when referring to allele?

Incompletely dominant means that the dominant allele does not completely rule over the less dominant allele in that the less dominant allele shows some through the more dominant one. Such as red and white flowers. In an incomplete dominance case, the red and white flowers would make pink flowers because the dominant allele (red) does not completely rule over the less dominant allele (White).

Incomplete Dominance is when neither allele is dominant over the other. As a result neither allele is comletely shown.
For example;
Snapdragons (flowers) come in both Red and White. When these two flowers 'reproduce' together, the alleles are 'incompletely dominant' and instead of the flower being red or white, the two colours mix to form a pink snapdragon flower.

Hope this is what you are after

Incompletely dominance is when an allele is not completely dominate over another allele.

For instance, If you cross a red flower (RR) with a white flower (rr),  you would get a PINK flower since the red flower is incompletely dominate (Sort of like "blending"). If this was regular dominance, if you cross RR with rr, you would automatically get a red flower.

However it cannot always be considered the same as "blending". Another example could arise when discussing sick cell anemia. Lets say S = sickle cell, and s = normal blood cells. If your are a homozygous dominant (SS) then you will have sickle cell; however, if you are a heterozygote, you don't necessary have sickle cell, but you may have mild symptoms. But if this was regular dominance, if you were a heterozygote you would have sickle because the sickle cell allele is dominant.

Overall, with incomplete dominance, just because an allele is "dominant" doesn't mean that that allele will be expressed more in the phenotype.

I hope that was clear, and didn't confuse you anymore!