'Wet rental rate' describes one pricing scheme for the rental of aircraft, usually small planes in general aviation. A 'wet' hourly rate means that the rate includes all consumables - usually fuel (either avgas or turbine fuel) and oil, perhaps oxygen if appropriate - associated with the use of the aircraft. Generally, facilities will offer either wet or dry rental rates, or a choice of the two.
Which is better for a particular renter will typically depend heavily on what type of flight they are making. For a trip, a wet rate may be advantageous because the renter can fly at otherwise-uneconomic speeds - above the 'most efficient cruise speed' - and get to their destination more quickly. This will result in a lower number of hours of Hobbs time (aircraft rental is usually done in 'hours of running engine time' which is measured using a device called - in the U.S. at least - a Hobbs meter).
If, however, the aircraft is being rented for local work - flight training, for example - a dry rate may be better, as it is possible that the aircraft will be run at lower-than-maximum power for much of the time (for example, while in the traffic pattern, power levels will be low once the climb out is complete). Thus, it might be possible to come out ahead.
Many flight schools will not offer a dry rate, both for business reasons and to prevent students from trying to 'minimize' on fuel burn or fuel loads during training, which may not be safe.