What is self assessment
Most people don't need to tell the tax authority HMRC what they earn. It's dealt with automatically thanks to reporting by employers and other ways of earning money.
But some people do need to report what they earn. The method for doing this is called self assessment. Originally it was a printed form where people would fill in what they earned and send it off to HMRC. Today, it's all handled within untied.
The main people who have to use self assessment are the self-employed, property landlords and those earning more than £100,000 through employment. See the related article on Do I need to file a tax return :-)