A coolant flush usually refers to changing the fluid in the radiator of your car and can mean having chemicals 'flushed' through the vehicle's cooling system so as to remove any unwanted rust or corrosion. Coolant is the greenish liquid that you find in the radiator and should be changed according to the owner's manual of your vehicle. As for cost, it depends upon who is doing the job, a dealer's service department will charge more than a garage; but they will make sure that the job is done correctly for your vehicle. In any event I would shop around to find a reputable mechanic who will give you the best price.