In the world of indoor plumbing, a wet vent is a connection between an air pipe (vent) and the main drain pipe (soil stack) that occurs lower on the soil stack than a connection between that same soil stack and any other drain (toilet, shower, sink). Ideally, the vent pipes, which equalize air pressure within the entire drain waste vent system, should connect to the soil stack above any drains so the vents don't get clogged up with waste, but this option may not be feasible for certain types of plumbing projects, such as adding a bathroom on a lower floor.
A wet vent is essentially a plumbing compromise that uses the main drain pipe as both a drain and a vent when access to the top of the soil stack is not available. The risk associated with a wet vent is that it may not equalize the air pressure within the pipes as well as a dry vent, so there is a chance that water rushing down the soil stack could either create a vacuum which sucks the water out of a trap, or build up enough positive pressure to expel the water out of a trap. Either situation is bad, because the water that sits in each curved trap forms a constant seal which prevents sewer gas from escaping up through your drains and into your house.