Pneumatic means "related to air or gas." If you've ever gone to a drive-through bank where you put your deposit in a container that gets shot through a tube into the bank, by a blast of air, then you've used a pneumatic tube.
When you pronounce pneumatic, keep the "p" silent. It comes from the Latin root for breath, and in some cases people call spiritual things "pneumatic." In engineering, when something is described as pneumatic, it usually uses air pressure to move things around, like pneumatic tubes, or like the compressor in the whipped cream that pushes air into the liquid cream as it comes out of the can.