The Dumbwaiter is a food elevator (lift) system that stops on two floors; the kitchen or the laboratory on the bottom floor. It isn't possible for Pajama Sam to ride the lift down to the laboratory as he would be bypassing the doors of knowledge, but it is possible to ride it from the laboratory to the kitchen. He is often shown sleeping when not in use.
The dumbwaiter appears in all versions of the game, but he is only important to the Darkness's Living Room storyline. In all versions of the game, Sam can't use the dumbwaiter to go down to the laboratory as it would deny him access. Sam can however use the dumbwaiter to go from the dumbwaiter to the kitchen.
In the Darkness's living room storyline, Sam will need to use the dumbwaiter after taking the invisibility potion in order to get to the living room and take his mask before the potion expires (as the potion is the only thing that will allow Sam to take his mask out of the living room without getting seen by the dancing furniture).