You can get away with not covering your garden furniture in light weather conditions. In more extreme weather, such as heavy downpours or high winds, you should cover your furniture to increase the life span. Aluminium frames are rust proof and rattan frames are incredibly hard wearing but soft furnishings like cushions should always be covered to stop mould.
The best cover to buy is a breathable furniture cover. Plastic and synthetic covers can produce moisture which can lead to mould and rot. Many garden furniture brands have covers specifically for the furniture set you are buying, but you can buy any cover as long as it is big enough to fully protect the furniture. Measure your furniture, barbecue or swing chair to ensure you get the right size.
You can keep garden furniture cushions under the cover with the rest of the furniture, or you can buy a separate cushion bag which can be stored indoors or in a shed. You can also buy cushion storage boxes made specifically for garden furniture cushions. They come in a range of sizes so check you get one big enough for your needs.
Garden furniture covers are not completely waterproof. The best covers are breathable and so do let air in. This means they are not totally waterproof and may allow small amounts of moisture to get in. If water is allowed to pool on top of the cover it can start to drip inside. Prop the cover up to form a pyramid shape and this will stop water settling and allow it to run off, reducing the risk of water damage.
If your garden furniture cover is broken or unusable, you can use a plastic sheet or tarpaulin as an emergency cover. This will stop any water getting in if it is raining and block sunlight in the short term. For long term use though, a dedicated garden furniture cover is the best option.