A bike crank arm is the part of the bike that transfers power to the chains. A pedal will almost always be mounted to a bike crank arm to help generate its energy. The chain ring is also bolted to the crank arm, and is the primary lever that you use to drive the chain with pedal action. This particular bike crank arm is 170 millimeters long, and they use a square taper-type bottom bracket which is made out of aluminum.
It's pretty common if you're peddling hard to pull the threads out where the pedal threads in, thus requiring a new crank arm.