"Non-destructive audio processing" means that the actual audio data in an audio file is not edited, only peripheral or playback parameters are changed.

To boost the level of a waveform to its maximum amount short of clipping (distortion). This maximizes resolution and minimizes certain types of noise.

The process of adjusting the volume level on tracks as they are encoded, so that their peak levels fall within a similar range.

Nyquist frequency
The highest frequency that can be reproduced accurately when a signal is digitally encoded at a given sample rate. Theoretically, the Nyquist frequency is half of the sampling rate. For example, when a digital recording uses a sampling rate of 11 kHz, the Nyquist frequency is 5.5 kHz. If a signal being sampled contains frequency components that are above the Nyquist limit, aliasing will be introduced in the digital representation of the signal unless those frequencies are filtered out prior to digital encoding; see aliasing, low-pass filter.