In Final Fantasy games, "Kupo" is basically, um, the thing that Moogles say, if I may be so lowbrow. The idea is that because Moogles are not human (they're merely anthrophiles) they must have some fundamental differences. These differences have, so far, been limited to appearance, outlook on like, and the word "Kupo".
Moogles were introduced to the series with Final Fantasy V, but at that point, they did not speak. One character in the game could understand them, but other than that, there was no implication that they could communicate.
In Final Fantasy VI, we had much more contact with Moogles. This was mostly in the form of the character Mog, but nonetheless, there were Moogles. And because they spoke, they needed to speak in a cutesy way. And thus was Kupo born.
Kupo is basically the equivalent of "smurf". As in:
- Emphasis: We should go kill that villain, kupo!
- Surprise: You got me a new-in-box SNES, kupo!
- Agreement: Let's dance, kupo! (Kupo!) (Kupo!) (Kupo!)
- Threat: Give me the kupo money or your friend the white mage gets it!
- As a name for all things Moogle: This is my friend Kupo. He lives in Kupo Village and enjoys eating Kupo Nuts.
- Exclamation: Oh, kupo! I spilled the Echo Screen all over the floor!
And so on and so forth. Kupo is essentially an all-purpose phrase. Were you to buy a stuffed Moogle, and it said something when you hugged it, that something would doubtless be "Kupo!". If you wanted to break into a Moogle's computer, his password would most likely be "kupo". If you wanted to compliment a Moogle, you probably would not offend him by smiling and telling him that he's looking very kupo today.