[W]hat is the Major allele [frequency]?
If the Minor Allele Frequency is $p$, then, for a bi-allelic locus, the major allele frequency is obviously $1-p$.
[W]hy reporting the second most frequent allele is helpful?
Most polymorphic loci of interest are bi-allelic. Hence, the MAF is an indication of genetic diversity. One could similarly report the ...