Income on which tax has already been paid
When you're calculating the tax due, you need to work out whether or not you have received any income on which tax has already been paid. For example, interest on savings in a building society or bank account usually has tax taken off before it's paid to you. Income from employment or an occupational pension should also have had tax taken off before the payment was made to you.
When you're working out the total tax due for the year, you need to take into account that tax has already been paid on this income. The gross amount of this income (that is, the amount you received plus the tax deducted) is taken into account when calculating your total taxable income before personal allowances and tax reliefs are taken off.