Allele specific depth. i.e. if ref is 'A' and alt is 'G' and AD is '6,9' you got 6 A reads and 9 G reads.

If this is produced by GATK look in the header output - it usually explains each flag.

EDIT:

Checked GATK, read depth is only those used in calling.

From the GATK forum:

AD and DP : Allele depth and depth of coverage.

These are complementary fields that represent two important ways of thinking about the depth of the data for this sample at this site.

AD is the unfiltered allele depth, i.e. the number of reads that support each of the reported alleles. All reads at the position (including reads that did not pass the variant caller’s filters) are included in this number, except reads that were considered uninformative. Reads are considered uninformative when they do not provide enough statistical evidence to support one allele over another.

DP is the filtered depth, at the sample level. This gives you the number of filtered reads that support each of the reported alleles. You can check the variant caller’s documentation to see which filters are applied by default. Only reads that passed the variant caller’s filters are included in this number. However, unlike the AD calculation, uninformative reads are included in DP.