Hummus is made from mashed chick peas, tahini (sesame seed paste), garlic, and lemon juice. You can add other flavorings to it, too, but that's basically what it is.
A lot of people use it as a dip, for chips, pita, or fresh vegetables. I also like to spread it on bread or a tortilla as a part of a sandwich or a wrap. It's pretty versatile, but if you haven't had it before, I'd start out with a store bought tub and some fresh veggies and see what you think. If you like it, you'll find it's pretty easy to make your own at home, but there are so many at the grocery store that you can save time buying it and it'll still taste good.