A conversion refers to a situation where a website visitor performs an action requested by the website operator (purchase, inquiry, newsletter subscription, registration, etc.). Closely related to conversion is the concept of Conversion Rate, which is obtained by dividing the number of conversions by the number of total ad interactions that can be attributed to a conversion during the same period.
For example, if you record 140 conversions out of 2000 interactions, the conversion rate will be 7%.