What does "quintain" mean?

Quintain: An object to be tilted at; -- called also quintel. [Written also quintin.] Note: A common form in the Middle Ages was an upright post, on the top of which turned a crosspiece, having on one end a broad board, and on the other a sand bag. The endeavor was to strike the board with the lance while riding under, and get away without being hit by the sand bag. "But a quintain, a mere lifeless block." Shak.