CIS applies to people who work in construction only.
Under the Construction Industry Scheme (CIS), contractors deduct money from a subcontractor’s payments and pass it to HMRC. The deductions count as advance payments towards the subcontractor’s tax and National Insurance.
Register for your UTR & CIS here

HMRC provide a personal allowance* of income before you eligible to pay tax. By completing a self-assessment tax return after the tax year closes may mean you are due a rebate.
*Personal allowance in Tax Year 18/19 – £11,850
*Personal allowance in Tax Year 19/20 – £12,500