Definition
The wet-bulb zero level is the the lowest level in a sounding at which the wet-bulb temperature is 0°C.
During the onset of a cool-season precipitation scenario, the higher the initial wet-bulb zero level, the less chance of the precipitation changing to freezing/frozen precipitation at the surface. Also, during convective season, lower wet-bulb zero levels can indicate a higher probability of hail occurrence.
Skew-T Procedure
From the surface, follow a 0°C isotherm upward until it crosses the wet-bulb temperature profile.
That level is the wet-bulb zero level.