In the UK everyone who earns an income is entitled to a tax-free personal allowance.
This is a set amount of money that can be earned, either through employment or other means (self-employment etc.) That will not be subjected to taxation.
For the 2019/2020 tax year this personal allowance is set at £12,500. Therefore, any income earned in the current tax year will be tax free until your total earnings reach £12,500. Once your earnings go over this amount they will be to be taxed at the rate of taxation based on your total earnings (for most people this is 20% unless you are a higher rate payer)
Personal Allowance Reductions
As soon as annual earnings go over £100,000 the personal allowance starts to be reduced: for every £2 earned over £100,000 the personal allowance is reduced by £1.
This means that anyone who earns £125,000 or above has no personal allowance.