Spaniards will eat 12 grapes when the bell rings on New Year's Day, what does it mean?

At midnight on December 31 every year, there will be many Spaniards gathered in Madrid's Sol Square waiting for the arrival of the New Year. They have a very interesting custom to welcome the New Year, that is, when the New Year's bell rings, start eating grapes, every knock, eat a grape, a *** knock 12, eat 12 grapes, it is said that in the new year, this will bring good luck, this custom has continued for hundreds of years.