It's a Japanese word that literally means "receive". It is pronounced "ooh-kay", with the accent on the first syllable.
In Aikido, we usually practice in pairs. One partner is called "nage" (or "tori" in some schools), and the other is called "uke".
Uke is the one who attacks, giving energy to nage that enables him or her to practice the technique.
We also call the practice of falling and rolling "ukemi", which decomposes to "receive with the body".
Uke is a role which I particularly enjoy, so these days that's where most of my email comes from. You may wish to read the relevant section from the Aikido West Beginners' Handbook.