What does it mean when it say employment is taxed in the tax year it is received

i have a question, i earned a salary of £35000 per year, i completed my accountancy exams in September 2019 and as such was entitled to a bonus of £2000 however my employer could not pay me the bonus due to financial difficulties and i eventually got it in July 2020……what tax year should i pay the tax on the bonus received and why