Tahini is a rather bitter paste of pulped sesame seeds, a bit like a thinner, non-peanutty version of peanut butter, usually without any salt added. It is an extremely rich source of vegan calcium and a good thing to spread on toast with marmite or marmalade or honey. It can also be added to a whole range of other foods which could stand being a bit more bitter and oily.
To make a tahini dip (tehina) thin it down with olive oil, lemon juice and water, add a bit of salt and garlic, and blend it well. Another use for tahini is making halva, by mixing it with syrup or honey and maybe adding some nuts and vanilla or cocoa and letting it set. But perhaps most importantly of all, tahini is an essential ingredient of houmous, food of the gods.
I have occasionally come across tahini made with toasted sesame seeds, giving it something of the flavour of sesame oil, and I highly recommend it if you can ever find a jar.