Cram was a biscuit-like substance made by the Men of Dale and the Lake-men, and shared by them with the Dwarves of Erebor. It was said to be nutritious, and was used as sustenance on long journeys, as for example by Thorin and Company on the last stage of their quest. When Gimli tasted lembas, he compared it to cram.
It was made from tightly pressed flour or meal. Basic cram like this was a flavourless, uninspiring food, but sometimes milk or honey would be added to make it a little more palatable.
In The Etymologies, cram is said to be a Noldorin word (the precursor of Sindarin), from a root (KRAB-) meaning "press". It was an "added entry"; Tolkien wrote much of The Etymologies before The Hobbit came out. It is unclear whether this was any more than fancy at the time, as several of these "added entries" were later reenvisioned with a non-Eldarin origin.[source?]
cram also has relevant meanings in English, like "To eat greedily, and to satiety; to stuff oneself".