A tax code is a series of letters and numbers that HM Revenue and Customs (HMRC) uses to calculate how much income tax should be deducted from an individual's pay or pension. The tax code is used by employers or pension providers to ensure that the correct amount of tax is deducted from an individual's income.

The tax code is based on the individual's personal allowance, which is the amount of income they can earn before they have to start paying income tax. The tax code may also include other factors, such as the individual's taxable benefits or allowances, and any deductions or adjustments that need to be made.

There are different tax codes for different types of income, such as employment income, pension income, or income from self-employment. The tax code may also be adjusted if the individual has other sources of income, such as rental income or investment income.

If an individual's circumstances change, such as if they start receiving taxable benefits or their income changes significantly, their tax code may need to be adjusted. This can be done automatically by HMRC or the individual may need to contact HMRC to request a new tax code.

It's important for individuals to check their tax code regularly to ensure that they are paying the correct amount of tax. If the tax code is incorrect, they may be paying too much or too little tax, which can result in a tax bill or a tax rebate at the end of the tax year.