A serpentine belt is the belt that generally runs all of the components under the hood that are belt driven. They can include power steering pump, water pump, alternator, A/C compressor, etc. It is a big deal. Simple item, easy to replace, very necessary as well. I would consider replacing it, especially if when looking at it you see any signs of wear, cracking, etc. Another thing that I do is take the old one and throw it on the spare tire to keep with me in case the new one should ever become defective on the road. It usually is enough to get me home. Maybe $50 for replacement and labor.