The word "allele" doesn't appear very often in articles about gardening. An allele is a particular form of a given gene. Recall that in the usual forms of most organisms, the chromosomes occur in pairs. Both the members of a pair have essentially the same sequence of genes, and they are at the same location or "locus" (plural, loci) along the paired chains. Usually the corresponding genes in each of the two chains are pretty much identical. They don't have to be identical, of course. In either case, each of those two genes is an "allele".
If the two alleles are in fact identical, the organisms is said to be "homozygous" at that genetic locus. If the two alleles are not identical, then the organism is "heterozygous" at that gene locus.