The VAT fraction is used to quickly calculate the VAT element of a price that is inclusive of VAT.
At the 20% rate the fraction is 1/6. Eg. If you fill your car with £60 of petrol the VAT you will have paid is £10.00 (£60 x 1/6).
At the reduced 5% rate the fraction is 1/21. Eg if you spent £80 on a reduced rate item in total, the VAT element is £3.81 (£80 x 1/21).