The advertiser graphical tool explained:

Hits = How many people with an IP address from that area, "hit" a site that served them an ad.

Percentage = what percentage of "hit" took place on any given day of the 7 day spread for that area.

The graph shows the last seven days, yesterday and the six days prior.

The counts are ads that either were or could have been served and the percentages are that days percentage of the weekly total.