What is a crankshaft sensor and is it really this expensive?

I know nothing about cars, that is why I bought a Honda Accord. I am being told my car needs a "crankshaft sensor" along with a new timing belt. $250 for the timming belt but $500 for the sensor...plus whatever labor ends up being (a fortune, I am sure). Can someone tell me honestly what a crankshaft sensor is and what it does. All they are telling me at the shop is that I need it and car won't run again without it. I honestly have no idea if I am getting ripped off or not...I am an payroll auditor, not a car person :( Thanks! Honda Accord EX 2.2L