This is the certificate issued by an employer or a representative employer which states the amount of income paid to you in a given tax year. The certificate also states the amount of taxes (PAYE) withheld from the income you received. This certificate is the proof you and the employer have for the amount of taxes withheld and paid over to Inland Revenue during a tax year.
PAYE is an abbreviation for Pay-As-You-Earn, which means that taxes are withheld and paid to Inland Revenue every time you earn income, i.e. normally on a monthly basis.
See below where you can find the certificate reference number on the actual certificate: