Hi Nana. A bumper crop means a unusually large crop growth and harvest, for example grain, fruit, vegetables. The word 'bumper' in the 17th century was a large glass of beer or wine that was filled to the brim, that was the sense of bumper meaning 'large'

In the US, farmers use the term "bumper crop" when they mean that their harvest is/will be larger than they expected. It is like having a large enough harvest to pay their bills and have money left over to save for the next time their crop fails.