Cellular Data is what you use in order to access the internet when your device is not connected to Wi-Fi. You can turn this feature on or off to determine whether or not you want your device to automatically use your cellular data to keep your device working. If you use cellular data, that is what you get charged for under your data usage that you pay for monthly.
You can turn cellular data on or off to limit apps and services from using the cellular network to connect to the Internet. When cellular data is on, apps and services use your cellular connection when Wi-Fi isn't available. As a result, you might be charged for using certain features and services. https://support.apple.com/en-us/HT201299