I contacted a website owner to buy media directly from them, they asked me what my fill rate is?
What exactly are they asking?
I believe fill rate = number of ad impressions served / number of page impressions.
A 100% fill rate would mean every visitor from every country is always shown an ad.
This is more of a network metric... i.e. a publisher signs up with some network and they place ad tags on their site. The network sells traffic to advertisers such as affiliates. We make campaigns and target specific geos, and our ads get shown on some persons site. Sometimes a visitor may come to the site, be from Tanzania, no campaigns are active to show ads to Tanzanians in whatever category the site falls into > no ad is shown. Down goes the fill rate.
For you, if you are going to be advertising to everyone, not geo-targeting using an adserver, then your fill rate would be 100%!
Make sure they realise you are a media buyer, not a network. If you are doing a flat buy your fill rate really shouldn't matter too much.