as mentioned by Rubal, MAF states for Minor Allele Frequency, but it is always critical to know not only which population or samples is referring to, but also which allele is referring to. this last part goes for non bi-allelic variants, whose MAF is usually reporting the frequency of the second allele, and not the minor one. we tend to think that all SNPs are bi-allelic, and most of them they are, but it's always a good idea to check which allele the MAF is exactly reporting.