An ashcan is a role-playing game that is released publicly while still in the last stages of work-in-progress, for the purpose of gathering feedback for final touches in a public beta playtest. Ashcans are generally sold for money (which can help limit the audience to only those interested in playing it) and many have production quality similar to finished, final products.
People who have bought an ashcan may also receive a discount on the final game when it is released.
The term ashcan comes from the comics industry, in which companies would produce a small run of cheaply made or unfinished comics for the purpose of legally retaining certain intellectual property (titles or characters, etc.). These weren't intended for sale, but rather for the ashcan, i.e. the trash.