If your guinea pigs live outdoors, they will need protection from the cold, wind, and rain, particularly during the winter months. A guinea pig hutch cover is designed specifically to provide this protection, in a way that won't endanger your animals.
A hutch cover is essentially a large fitted plastic sheet which encloses all sides of the cage. Some have features such as double thickness walls or a special lining, but the most important thing is that you get a cover which is waterproof. There are a couple of different types of cover, some which just sit loosely over the hutch, and others which have built in straps. These are considered better because they allow you to tighten the cover over the hutch, providing a snug fit, and reducing the chance that the cover will come loose in the wind.
The front panel of a hutch cover is made from transparent plastic, so that you can see in to your pets and they can see out. This panel should contain ventillation holes to ensure a good supply of fresh air to the hutch. This window can be unzipped and lifted to provide access to your pets, so that you can refill their food and water, clean the hutch, and get them out to play, all without needing to remove the cover.
You can buy covers in a range of different sizes, so be sure to purchase the one which is appropriate for your hutch. Many cage manufacturers also sell covers for their products - these are ideal because you can be sure of a perfect fit.
If you don't have a cover and need to provide your pets with immediate protection, you can make a makeshift cover simply by hanging an old rug over the hutch. However, this is far from ideal, and should not be used as a long-term solution - invest in a proper cover instead.
A hutch cover on its own will certainly help to protect your animals against the elements, but a far better solution is to move them to a more hospitable place. Letting them live inside your house is the best solution, and provides them with a number of benefits in addition to keeping them warm and dry. However, if this is not possible, see if you can at least move them into an outbuilding such as a shed or garage.
A guinea pig hutch cover provides your pets with the essential protection they need during the winter months. As their owner, it is your responsibility to ensure they have the best living conditions possible, so be sure to invest in the best cover you can find.