The Ancestry V2 chip is Ancestry.com's second microarray chip. It is a custom version of the Illumina OmniExpress Plus Genotyping BeadChip. It's their current chip.
Microarray chips test a small percentage of SNPs on the human genome. Each letter the DNA code for the human genome (A, C, T, or G) is called a base pair. This one tested about 669,000 SNP markers.
SNPs per Million Base Pairs – Heatmap
The V2 chip tests 669,000 base pairs across the human genome. This heatmap shows how those SNPs are distributed across the 22 autosomal chromosomes and the X-chromosome. The ranges are for counts of tested SNPs in blocks of 1 million base pairs.