tonneau cover refers to two different, (but similar), things in the automotive
One meaning for tonneau cover is for a leather or vinyl covering. That snaps in place covering the interior of a convertible when the roof is down. This will usually feature a zipper or some other way for the driver to operate the vehicle with the passenger area still covered.
The second, (and recently more common) meaning for tonneau cover is something that easily covers over the cargo area of a truck bed. This can range from a soft $30 universal vinyl covering, to a $1000 color matched aluminum model, with invisible mounting hardware.