Unleveled vs Leveled RF Power
In a nutshell, a (RF) signal generator that has a "levelled" output means that the output level remains constant as the frequency changes.  The generator contains active circuitry (AGC) that adjusts the output level so as to keep it constant.

An "unlevelled" generator does not have such circuitry and the output level changes with frequency.  This output level variation can be quite significant.