What is low level modulation?

Modulation of a signal at a point in a communications system or device where the power level is low, such as at a distribution frame or a radio microphone, compared with the output power, such as the output power of a long-distance line driver or the output power of a radio transmitter antenna.

What is low level modulation and high level modulation?

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.