A multiple allele trait is one of at least three different types of a specific gene, but only two of these types are able to exist in a diploid. A diploid is a type of organism or cell that has two sets of chromosomes, with one set typically from the father and the other from the mother.
An allele is a gene alternative on a specific chromosome and in a specific location. In humans, who are diploids, offspring inherit an allele from their mom and another allele from their dad. The organism's phenotype depends on both of these alleles. Phenotype determines what an organism, such as a human, looks like and includes things like hair color, skin color, eye color and other physical characteristics.
Alleles are either recessive or dominant. Dominant alleles are essentially more forceful and produce the same physical characteristic whether its partner allele is different or identical. A recessive allele needs its partner allele to be identical or else the phenotype is not apparent. To put it simply, a dominant allele is very independent and can do the job of producing a phenotype on its own, but a recessive allele needs some help and requires an identical partner to get the job done and produce a phenotype.