in Analog modulation system, the AF signal is modulated by a High-frequency carrier oscillator signal. This is Low-level modulation. After this modulation, the resultant modulated signal will be applied to the input of a RF power amplifier. Just after this amplification, a second-stage of modulation is performed with another carrier-oscillator. This is High-level modulation. After this the signal will be sent out for transmission.
So, Low-level modulation is done on a signal whose power-level is low and High-level modulation is done on the signal whose power level is high.
Hope this helps.