A tax code is a series of numbers and letters, for example 1257L, S1257L, C1257L and K497. If you're a full or part-time employee these codes are used by your employer to calculate the amount of tax they need to deduct from your wages before it's paid into your bank account. If you have a pension, you will also be given a tax code with any money over the applicable tax threshold being eligible for taxation.
Your tax code will usually be made up of letters and numbers. The letters and numbers indicate different things. Numbers show the amount you can earn tax-free. This is called your personal allowance and is found by adding a zero to get the real number. For example, 1257 means you can earn £12,570 a year tax-free.
The letters relate to your situation and how it alters your personal allowance. There are many different letters. For a full guide check out the UK Government website.