Cloud storage is an alternative to storing all your data (your documents, images, videos etc) on your own phone, tablet computer or external hard drive. Instead, you upload them to a cloud storage service which stores them for you. This allows you to access them anywhere you have an internet connection, on pretty much any device. It also means that if your devices are lost, damaged or stolen, you'll still be able to retrieve your data.
These services often offer extra features to tempt you to buy. These may include automatic backup of your files, facial recognition to make sorting your photos easier, and security features to keep your data safe.