wat are they for exactly b/c mechanic said i need them put in b/c my rear breaks were really ****ed up and they cost me 229 for both?

 

I assume your talking about brake calipers, I'm sure you know what brakes are for, I suggest you pick up a Haynes manual for your Maxima, worth it's weight in gold, especially for someone like yourself. Good luck

 

The brake caliper is the part that squeezes the brake pads together forcing them into the brake rotor causing friction which in turn stops your car.

The rear calipers are somewhat expensive at about $70-80 for each side because they are more complicated mechanically than the front calipers. Did your mechanic include new brake pads as well? If so, $229 was a pretty good price for all the work and parts.