Fan belts are important to cars. During the colder months, we see (and hear) more squealing fan belts. Wear and tear will eventually loosen all belts but in colder weather, metal engines and parts shrink a bit, causing loosening.
A squealing belt is you car's way of warning you of problems and you should come in right away to have it looked it. In the old days, a car three or four fan belts, each one powering the alternator, water pump, air conditioner or power steering. If one belt broke, chances are you could at least make it to the next service station for repairs.
Most newer cars just have one belt, called the serpentine belt, that threads through the engine and powers all these things. It's a more efficient system but if that belt breaks, the car will likely fail immediately due to loss of function to multiple systems. In addition to all the noise, a belt about to fail will also show signs of wear.
We can fix loose belts. Sometimes the belt needs replaced, while other times a spring-loaded tensioner is failing. Whatever the cause, we can fix belts quicky and they're usually not a high-cost repair - but they are high risk if not attended to.