Domain tasting is the abusive practice of domain name investors who purchase a domain name with the intent of testing its popularity — that is its existing traffic levels. If it meets a minimum criteria the investor keeps the domain. If it does not appear to have traffic value, it is returned to the registrar using the five-day grace period that registrars provide.
Think about people buying Willie Wonka chocolate bars, opening the bars up to look and see if there is a golden ticket, and then returning the chocolate bar if there is no golden ticket inside.
The popularity of domain cannot be determined until a person has control of it and the daily number of visitors can be measured.