My serpentine belt snapped this morning. When I went to replace it, I realized that this part was completely loose and rattling around. Can you please tell me what it is and if it is easily and safely replaceable by an amateur with very limited repair experience?
That's an idler pulley with a defective bearing. The bearing comes already pressed into the pulley. You just remove the center bolt to remove the pulley, then bolt the new one on. I can't tell for sure from the photo, but this could be the spring-loaded tensioner pulley. They can be repaired the same way but they come as part of the entire assembly. Instead of removing the bolt from the pulley, you remove the bolt the assembly pivots on. When you install the new one, there will be a locating notch to hold it in the proper position as you tighten the bolt. Once it's installed, use a wrench on the pulley bolt to swing the pulley down to allow you to slide the belt over it.