This is the sacred black handled knife. It's used to focus and direct energy, for casting the circle, for invocations, and for consecrations; never to cut (in our tradition. Different traditions are different.) It is the symbol of the male, and of active energy, that is sent out into the world. Many consider it the most sacred tool of the craft. It's also aligned with the element of air, and is one of the symbols of air on the altar.
The athame is traditionally double bladed, to show that every spiritual truth cuts both ways, and that any energy used will come back, so be careful.
It's pronounced differently in different parts of the country; but usually it's ath-a-may, a-tha-may or a-tham-ee.