Mono = monophonic, i.e. only one sound plays at a time
Poly = polyphonic, i.e. multiple sounds can play on top of each other.
In an MPC, polyphony can be set on a program-wide basis, which controls the way all the pads in a program interact with each other.
Separately it can also be set on a per-pad basis, but this only controls how that particular pad interacts with itself.
Example 1: Drum KitAssume you have a simple drum kit:
A1 = kick, A2 = snare, A3 = closed hat, A4 = open hat
For a natural sounding drum kit, set the entire program to POLY. This way, by default, all pads will 'overlap' each other, just like a real drum kit would - i.e. play the kick followed by a snare and you'll still hear the kick tail playing underneath the snare.
Now look at each individual pad. In the MPC Software, the default pad play setting is MONO. This means if you play that same kick on A1 three times quickly in succession, each subsequent play will cut out the previously playing kick, so there's no overlap of those three kicks. This setting is independent of any other pad, so if you quickly played a hi hat on top, it would still overlap the kick (thanks to the program POLY setting).
Setting pad play to POLY (default setting for all other MPCs) means three successive hits of A1 will overlap, which for a booming 808 kick would probably cause gain issues. However for a 'natural' drum kit, I tend to prefer setting individual pads to POLY as it often sounds more realistic, because in real life you would still hear some reverberation from that original kick and leaving that overlap tends to mimic this - MONO just doesn't sound right in many cases. Another example is cymbals - setting a ride pad to MONO just sounds awful when you're playing a ride pattern with it, it's much better to hear it overlap itself (IMO).
The other thing to look at are the hats. In reality, the open hat on A4 is actually the same instrument as the closed hat on A3, so with your open hat playing, any instant of the closed hat should immediately cut off that open hat - this mimics the opening and closing of the actual hat.
To do this, you can use MUTE GROUPS. Simply set A3 and A4 to MUTE GROUP 1. This is a two way mute - i.e. A3 mutes A4 and A4 mutes A3. If you prefer a one way mute, use MUTE TARGETS.
Example 2: Bass ProgramMost bass lines sound better if the bass sounds do not overlap, so if you have a bass instrument program, simply set the whole program to MONO play and one one bass note can ever be heard.
Example 3: Chopped break programIf you've chopped a break into 16 equal slices, you'll usually want to set the program play to MONO to ensure those chops do not overlap. However the problem with this is that if you also chopped out an individual snare and then wanted to overlay some ghost snares and rolls, those rolls would also be affected by the program MONO setting and would therefore cut out the main break completely. There is no way to override the program wide MONO setting on a per-pad basis, unless....
Workaround: have your snare on a separate program, or more conveniently, set your entire program back to POLY and then set all the pads to MUTE GROUP 1 apart from your individual snare. With all pads set to the same mute group, the effect is identical to MONO program play, however this way you get to opt out' certain pads from being included.
I go into this in more detail with lots of practical examples in my 'MPC: Sampling Laid Bare' book:
http://www.mpc-samples.com/product.php/ ... laid-bare/