Layering is the process of stacking multiple sounds on top of each other to achieve the desired result. This is usually achieved by playing multiple synths/samples at the same time.
The Process of layering two or more kicks is known as kick layering. This is usually done to get different elements of the kicks into a single kick. For Example, a Kick with a good low end might be layered with a kick a fast attack to produce a more dynamic and punchy kick.
Chord Layering is the process of layering multiple synths to get the desired sound as often a single oscillator is not enough to produce the desired sound.
Bass Layering is the process of layering two or more synths so that they can cover all bass frequencies.
Note: Equalization is important while layering so that the frequencies of the different synths dont interfere.