From Leopard Gecko Wiki
When a Leopard Gecko is heterozygous for a trait, it means it has one copy of the gene. An example of a Leopard Gecko who is heterozygous for a trait is a Mack Snow. In all reality, Mack Snows are heterozygous to be Super Snows. You need two of the Mack Snow trait (homozygous) to create a Super Snow.
When using the letters on the Punnent Square, a heterozygous trait would be represented by a lowercase letter and a uppercase letter or two different letters both different cases.