We are looking for the phenotype of the known parent. The Punnett square tells us the known parent's genotype is Ww.
Note: The genotype is the set of genes for a certain trait of an individual, while the phenotype is the displayed characteristics of that trait. In this case, the possible genotypes are WW, Ww, and ww, and the phenotypes of those genotypes are long-winged, long-winged, and short-winged, respectively.
Also note: Genotypes with a dominant allele will always have a phenotype that displays the dominant trait.
This means our two alleles are W (long-winged) and w (short-winged). The former is dominant and the latter is recessive.
As stated, the genotype of the known parent is Ww; it contains a dominant allele. Since dominant traits are always displayed, and the dominant allele is long-winged, the phenotype of the known parent is long-winged.