The Tomatometer measures the percentage of Approved Tomatometer Critics who recommend a certain movie -- or the number of good reviews divided by the total number of reviews.
Back in the days of the open theaters, when a play was particularly atrocious, the audience expressed their dissatisfaction by not only booing and hissing at the stage, but throwing whatever was at hand -- vegetables and fruits included.
A good review is denoted by a FRESH red tomato. A bad review is denoted by a ROTTEN green tomato. In order for a movie to receive an overall rating of Fresh, the reading on the Tomatometer for that movie must be at least 60%.