Tahini is a paste made from toasted ground sesame seeds. It has the consistency of dense peanut butter. When mixed with water, it has the tendency to seize up like concrete. This is a good thing, as the further thinning with water results in a creamy sauce. Nutritionally, when added to cooked beans (as in hummus), the resulting combination of proteins is every bit as complex as that of meat.
Find it in the World food aisle or wholefoods section of your supermarket (though my own Tesco never seems to know where to put it, and keeps moving all over the place). Beware of similarly named products (usually in plastic tubs from the Middle East) which claim to be tahini but are in fact a ready-made sauce.