The convective condensation level (CCL) represents the height (or pressure) where an air parcel becomes saturated when heated from below and lifted adiabatically due to buoyancy.
In the atmosphere, assuming a constant water vapor mixing ratio, the dew point temperature (the temperature where the relative humidity is 100%) decreases with increasing height because the pressure of the atmosphere decreases with height.
The CCL is determined by plotting the dew point (100%RH) versus altitude and locating the intersection with the actual measured temperature sounding.
It marks where the cloud base begins when air is heated from below to the convective temperature, without mechanical lift.
[1] Once the CCL is determined, the surface temperature necessary to raise a mass of air to that height can be found by using the Dry Adiabatic Lapse Rate (DALR) to determine the potential temperature.