Allele frequency (AF) is a measure of how common a particular allele is within a population. It is calculated as the ratio of the number of times an allele is observed to the total number of alleles at that locus (e.g., an AF of 0.5 indicates the allele is present in 50% of the population). Allele frequency helps identify rare and common variants and is a key metric in population genetics, disease association studies, and variant interpretation. In VCF files, AF values provide insights into the prevalence of observed variants across different samples.