In music and audio, "chorus" refers to when multiple sound sources (or people) play (or sing) the same notes together in unison. If two or more sound sources play the exact same frequencies and waveforms as each other-- e.g., two perfect sine waves at precisely 440.0 Hz-- the waves will interfere with each other, either reinforcing each other or canceling each other out, due to how the phases of the two waves relate to each other and how they're "added together." In theory, two waves can even completely cancel each other out and result in silence, but in practice this never(?) happens because of variations in the frequencies and sound waves which are being produced by the multiple sound sources. In any case, when two or more instruments/people play/sing in unison, the resulting sound is "richer" or more "full-bodied" than the sound produced by a single instrument/person.
Electronic keyboards, synthesizers, or VST plug-ins can use a "chorus" effect to make their sounds "fatter" or "richer." Different methods can be used to simulate the chorus effect, often depending on the type of instrument-- i.e., a synthesizer might use two or more detuned oscillators, whereas a "ROMpler" probably uses another method. An electronic keyboard will generally have two kinds of "chorus" parameters-- one is the actual "chorus type" (i.e., the keyboard may offer a variety of chorus types for you to choose from), and the other is the "chorus depth" or "chorus send level" which determines how strongly the chorus effect is applied to each voice (or tone, patch, etc.-- whatever your keyboard's manufacturer calls the various instrument sounds that the keyboard can play). When you layer different sounds together, or split the keyboard between different sounds, you may or may not be able to pick a different chorus type for each voice, depending on the keyboard's capabilities-- i.e., you might be limited to picking just one chorus type which will be applied to all of the voices in the layer or split. But even if you're limited to only one chorus type at a time, you should be able to control how much of the chorus effect is applied to each voice-- e.g., if you're layering an Electric Piano sound and a Violin sound together, you could have the chorus level turned all the way up on the Electric Piano yet turned all the way down on the Violin, or vice versa.
If the default Piano patch that you're using sounds pretty good, you might want to just leave it set as is. But you may also want to check the manual to see how to turn the chorus level up or down, so you can try out different settings. If you want the Grand Piano to sound "crystal clear," you can try turning the chorus level all the way down. On the other hand, an Electric Piano often sounds better ("richer" and more "shimmering") with the chorus level turned up. And once you've seen how changing the chorus level affects the Piano sound, you might also want to try out the various chorus types that your keyboard offers-- although you'll probably want to have the chorus level turned up all the way while you're doing that, to help you to better hear the differences between the various chorus types.
