Well assuming a moth board is something to do with butterflies........
A heat sink is a rather handy cooling device that takes heat away from a processor thus enabling it to function better

How does it do this?
Well it transfers heat from your processor through a thermal interface, usually silver or ceramic paste or a heatpad into your heatsink. Heatsinks are usually made of copper or aluminium, because of their high thermal conductivity. A fan then withdraws this heat from the heatsink, and with a well ventilated sytem inlet and exhaust fan(s) pass cool air in and hot air out.
Without a heat sink a processor will overheat and fail in a matter of seconds. Some have been known to catch fire.
The chipset on your motherboard and your graphics card wil have heatsinks too, as does some high end RAM.