The annual allowance is set by HMRC. It is the maximum your pension can increase by in a tax year before you receive a tax charge. All your benefits in the fund, and any AVCs and contributions to personal pensions or stakeholder arrangements, are added together each year to work out your pension input amount (PIA). This is tested against the annual allowance for each tax year.
The annual allowance for 2020/2021 tax year is £40,000.