A grapefruit was a citrus fruit native to Earth. The fruit was used for transporter experimentation due to its small size, lack of complex detail, and that the fruit wasn't a living being. (Star Trek)
In 2258, Montgomery Scott said that transporting a grapefruit from one planet to another in the same solar system vie transwarp beaming was easy. However, according to Scott, transporting a larger item, such as a dog or a Human, was more tricky.