What is the meaning of "no. of year purchase" at the time of valuation of goodwill

The valuation of goodwill is often based on the customs of the trade and generally calculated as number of year's purchase of average profits or super-profits. ... After calculatin...

it means expectation of purchasing firm to generate same operating profits from the business of vendor firm in future

see this example

Number of years' purchase is the number of years' for which the firm will expect to earn the same amount of profit because of the past efforts of the firm after change of ownership