Your Internet data is just like your internet credit i.e. you use it to browse the web. To access internet on your smartphone, you just need to convert your airtime credit in to Internet data (internet credit) by subscribing to any of the MTN data bundles.
Just as your airtime credit reduces each time you make a call, your internet credit (internet data) also reduces each time you visit a website, stream, upload and download files from a website.
Just as you need a certain amount of credit to make a certain duration of call, you also need a certain amount of internet data to be able to download, upload and stream a certain file from the web.
If you wish to download a 2MB file from the web,you will need 2MB internet data and so on.