Imagine researchers conduct a GWAS and find a particular allele (let's call it "T-allele") of a genetic marker (a SNP) that's associated with height. People who possess the T-allele at this SNP are, on average, taller than those who don't. After analyzing the data, researchers find that the T-allele is associated with an average increase in height of 2 centimeters (cm). In simple terms: If you have the T-allele, you might be, on AVERAGE, 2 cm taller than if you didn't have that allele. This 2 cm is the effect size of the T-allele for height. However, remember that height is influenced by many genetic factors and environmental factors (like nutrition). So, even if you possess the T-allele, it doesn't guarantee you'll be exactly 2 cm taller than if you didn't. It just gives an average measure of the effect of that specific allele.
