what is a crank sensor and what does it do

Hi I have a 95 Eagle Talon and just got word from a mechanic that is working on my ESi that there maybe a problem with the crank sensor can anyone tell me what a crank sensor is and what it does?

Its a sensor that picks up a trigger plate mounted to the crank. Basically just tells the ECU where the crank is in its rotation, and how fast its rotating (RPM).