What is low and high level Modulation

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.

For AM only
The most important aspect is that is if low level modulation is used all the resultant RF amplification stages must be linear. High level modulation is the variation of the power into a non linear RF power amplifier.
Low level :- low modulation signal power, lots of linear RF amplifiers.
High level :- lots of non-linear Rf amplifiers, lots of modulation amplifiers with a high power output (1/2 carrier power). Not for SSB.
For FM/FSK/Digital only low level modulation as RF amps can be non linear.
Frank