This is the face of your future communication with HMRC on any kind of tax. They would like us all to communicate online, via a GOVERNMENT GATEWAY NUMBER.
From there you can set up an online account called a Personal Tax Account, so you can access at any time your tax position.
You can also use it to register for self-employment, including the range of taxes that may then be applicable. You can monitor these taxes through your Business Tax Account, including making your regular payments.