It's simply a device that emits fluid in such a manner that there's no net force. Simple example, a T-shaped pipe where gas travels up the stem of the T and exits out both sides of the top bar.
A propulsion example: Shuttle would dump excess propellant by firing opposing pairs of RCS jets.
Shuttle's flash evaporator had vents on opposite sides of the aft boat-tail for the same reason.
Here's a Shuttle airlock schematic showing the non-propulsive vent for the External Airlock depress valve.
(this schematic is from the Shuttle ECLSS training manual. This book is online here but when they redrew this image to remove the internal airlock they screwed it up (it's figure 6-19 in the online version) This is a scan of my paper copy which shows the vent properly.)