It's a paste made from sesame seeds, and is an important element of Houmus (if that's how it's spelt). You can get it in most supermarkets, or whole food shops.
When you get your tahini, try baba ganouj! That's well-roasted eggplant[aubergine] with tahini, garlic, and lemon juice. Fresh mint optional.
I love the stuff so much that I quadrupled my eggplant plantings.
Tahini mixed with plain yogurt as a salad dressing is divine.