The wet-bulb temperature is a measure of moisture evaporation. Basically how easy it is for moisture to evaporate.
Wet-bulb is measured by an ordinary thermometer with a wet cloth on the end. You can also use a sling psychrometer or digital psychrometer as well.
When the air passes through the wet cloth, moisture from the cloth evaporates into the air. This causes the air to reduce temperature. The temperature the air has reduced to is the new temperature the thermometer will read, and this will be lower than the dry-bulb temperature in most cases. If they’re the same, you have 100% relative humidity.