Your vacuum cleaner's bag is for collecting particles, dust and fibres that have been sucked up by your device.
Make sure you replace the pouch, the bag on your cannister hoover or the receptacle on your upright hoover, especially before it gets over full. If you don't, your device may lose suction power or may even end up with a damaged motor due to dust getting into it.
You can order bags for your vacuum cleaner on our site. Many ranges are available depending on the model.