The personal allowance is a tax-free amount that any UK taxpayer can earn. The rate of the personal allowance varies from year to year. For the 2019/20 tax year, the amount you can earn tax-free is £12,500.
Under the PAYE scheme (applicable if you’re an employee) the personal tax-free allowance is divided in 12 equal portions, equivalent to the 12 months of the year. Any monthly earnings over the 12th of the personal allowance is normally liable to income tax. Personal tax allowances can be checked here.