A sales (or visitor) conversion is the event associated with a site visitor converting to a customer. That is, they stopped by just to check out your site and then decided to buy a product or service (or, for non-sales sites, participate as a committed repeat visitor). Conversion rates are notoriously low - it is not unusual to have a hundred visitors for every conversion.
Every effort should be made to construct your site in a manner which increases the number of conversions: good "grabber" copy, "buy now" incentives and so forth. The analogy here is that you are trying to cast your net with a finer mesh - fewer prospects get away.
However, in highly competitive markets, even highly optimized sites can continue to receive low conversion rates. Your only recourse is to attempt to cast your net wider - get more traffic.