I see this term used a lot. What exactly does it mean..?

A 'reward' can come in different forms:

One can be rewarded with a better technique
with a feeling of accomplishment
with the satisfaction and applause of the audience
with a self proclaimed success

In any case of the above I think the term 'rewarding pieces' applies loosly.

For me the idea is that you get to play something that pleases you so much and you gain so much on a sentimental and logical level that it's becoming very rewarding to do so. A work that teaches you A LOT, a piece that you cherish so much that you're dreaming of it while asleep (and awake sometimes).

I guess pieces of gold can be rewarding.

Where do you see this term "used a lot"? If you are referring to it in a musical context, I don't think I've ever seen it.

Regards,

Something different to everyone.

Pieces of eight.

A rewarding piece is a difficult piece for the player that takes a lot of effort to learn. The difficulty of the piece in itself isn't the issue, but it is the difficulty to the player. It doesn't matter if the piece technically is "easy" if the player finds it hard.

The player gets an enormous sense of achievement after perfectly playing/learning one of these pieces.

For me, a rewarding piece is one where the rewards are far, far greater than the effort it took to learn it (even if it took me one year to learn it). It doesn't have to be technically difficult (though it usually is); it just has to be one where I want to play it again and again, and it never palls on repeated playing.

One such piece for me is the Schumann/Liszt Widmung.

Worth the effort it takes to learn them.

As an aging amateur pianist, the idea of "rewarding" is the highest on my list of reasons to tackle a new work. I must have a reasonable sense that the emotional and esthetic payoffs in a work is commensurate with the technical demands required to achieve a satisfying level of proficiency. Beyond that, I really don't think you can be more specific.