Memberberries are sentient fruit first used by Mr. Mackey in his office. Later, Stephen Stotch shows them to Randy Marsh. Their main function is to remember pop culture icons, bringing about nostalgic feelings for the supposed good times of the past.
However, they also make homophobic and racist comments, such as remembering when marriage was only between a man and a woman, when there weren't so many Mexicans, and Ronald Reagan.
They appear as normal looking grapes with faces on each piece and speak in a very high-pitched voice.
The memberberries are the physical manifestation of the idiom "sour grapes", used to refer to a negative attitude to something because they cannot have it themselves.