The self assessment is a system that requires individuals to report their own income and tax liability to HM Revenue and Customs (HMRC). Self assessment applies to individuals who are self-employed, as well as to some other categories of taxpayers, such as company directors and certain professionals.

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Under the self assessment system, individuals must complete an annual tax return, which is a form that asks for information about their income, expenses, and other financial details. The tax return must be submitted to HMRC by a certain deadline, typically in January of the year following the tax year (the tax year runs from April 6 to April 5 of the following year).

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If an individual's tax liability is not fully paid through the tax withheld from their salary or other income, they may need to make additional payments to HMRC. These payments are known as "payments on account," and they are typically due in July and January.

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If an individual has overpaid their tax during the tax year, they may be entitled to a tax refund. In this case, they will receive a refund from HMRC after their tax return has been processed.