Gain staging is the process of controlling the level of audio signals throughout the various stages of the audio recording and mixing process. The objective of gain staging is to ensure that each stage of the process operates within its optimal dynamic range, avoiding both distortion and excessive noise. This results in a clearer, more balanced, and more professional-sounding recording.

The concept of gain staging is simple: at each stage of the recording and mixing process, the audio signal is given an appropriate level boost or reduction (“gain”) so that it can be properly processed by the next stage. 

The best example of proper gain staging occurs at the input stage, where an audio source, such as a microphone, is connected to a microphone preamp (either external or in an audio interface). Microphones output a very weak signal, which is defined as microphone level. A microphone preamp is used to amplify the microphone's signal to line level, which is a stronger signal and the optimal level for audio recording. When using a microphone preamp, it is possible to amplify the microphone signal so much that the signal becomes distorted due to clipping. A signal that is too loud can also cause issues when attempting to process the signal after recording, as there is no headroom to, for example, boost certain frequencies using an equalizer without clipping.

On the other hand, it is also possible to not amplify the signal to a high enough level, introducing noise to the signal due to the noise floor of the preamp and/or audio interface. If this signal is processed by something, such as a compressor, this can exaggerate and boost the noise introduced by the noise floor in the recording. In reality, due to modern technology and equipment, noise floor issues are rare, but it’s helpful in understanding why a healthy level and good gain staging are important.

Overall, a good level to aim for when recording digitally is -12dBFS; this corresponds to the -12 marker in most DAW meters. A signal of this level leaves enough headroom, and is loud enough to avoid any noise issues. 

These concepts can and should also be applied when using plug-ins or other outboard recording gear. For example, if you process a vocal with a compressor, you may need to adjust the compressor's output to compensate for compression. If you reduce the overall level of the vocal when compressing, you should boost the compressor's output to compensate for this reduction. You can usually see level changes after using a processor on your DAW's meters, as well as hear them.