According to the Grand Robert dictionary, the French dictionary of reference, « ratatouille » in its modern, everyday sense means:
Mod., cour. Plat d'origine niçoise, mélange de courgettes, de tomates, d'aubergines, etc. cuites ensemble à l'huile.
It's a vegetable dish that originated in the Nice area in southeastern France, and it's a mixture of zucchini, tomatoes, eggplant, and other summer vegetables cooked together in oil. Olive oil, of course. Other vegetables that go into ratatouille are red and/or green bell peppers, onions, and garlic.
Ratatouille niçoise in the early stages of cooking
Pronounce ratatouille this way: [rah-tah-TOO-yuh], with the stress on the third syllable. It's easier to make than to say, maybe. Slice or dice an onion or two. Roughly chop some fresh ripe tomatoes. Seed and cut up some bell peppers. Peel (or don't bother to peel) some zucchini and eggplant, and then slice or chop those up. Add a couple of cloves of garlic.
Cook everything in a big pot with olive oil, thyme, salt, and pepper. Let it cook until the vegetables are soft. Some recipes say you can mash everything together. Others say you want to be able to detect the individual pieces all the vegetables. Eat the ratatouille hot or cold, depending on the weather and what you're having with it.