That's a measure from Yahoo as to how many times someone is looking for a certain word or phrase. The higher the number the better. But remember this metric does not measure google or msn and the other search engines.
It's a good starting point but use it carefully. I've got domains with ovt scores over 300,000 and they get little natural type-in traffic, but others with a very small ovt score of 3500 get dozens of type-ins per day. Much of that has to do with domain's extension imho. High OVT domains usually do very well as optimized websites versus just parking them with a parking company.
As well, the results don't always properly reflect how a real person searches for a term. You will see the singular when they may be more often looking for the plural, or the word phrase may be jumbled around on the results page.