Diploidy: the act of having two sets of chromosomes. This occurs especially in somatic cells.
Diploid organisms are ones that have two copies of each chromosome in their cells (i.e., they have homologous chromosomes).
This becomes significant in a heterozygous individual.
Remember that an individual heterozygous for a particular trait is one with two different alleles for that trait at a particular gene locus (as illustrated below).
This allows the recessive allele to “hide” from natural selection.
As a result, variation persists in the population since it is not “selected” out.
Haploid organisms—ones that don't have homologous chromosomes—don't have as much variability because every allele is expressed and therefore can be acted on by natural selection.