What does heterozygous mean?

Alleles are different forms of the same gene.

If a diploid organism is heterozygous for a trait then it has two different alleles - eg Tt.

If it is homozygous for a trait then it has the two alleleles the same - eg.TT or tt.

In diploid organisms, e.g. humans, there are two copies of each chromosome in each somatic (body) cell. Each copy of the same chromosome carries the same genes at a specific point on the chromosome called a locus (pl. loci). Therefore, diploid organisms have two of each gene loci. The same gene can come in different forms called alleles. If a diploid organism has two different alleles of the same gene they are heterozygous.

A heterozygous gene is two of the same form of a gene—one from the mother and the other from the father. i.e. inheriting different eye color gene forms from mother and father result in a heterozygous gene.