A cranberry morpheme is a morpheme that only appears in one word, and whose meaning by itself is unclear or unknown to the everyday speaker.
They are named after the word 'cranberry' which is an example of the phenomenon: The morpheme 'berry' indicates that the word refers to a kind of small fruit (or whatever a berry is). However the morpheme 'cran' by itself is dead and doesn't mean anything, as opposed to, say, the blue in 'blueberry' or even the goose in 'gooseberry'.