If something is plentiful, there is a lot of it. Also, plentiful things yield large amounts of something, like an abundant farm. Apples are plentiful in a plentiful year on an apple farm.
Plentiful is an adjective to describe “many” or “a lot.” It’s plenty for “very much” and -ful for “full of” so plentiful is full of very much! It’s a word for “large amounts.” If you have plenty of potato chips, you have a plentiful supply of chips. Also, something that has or produces great amounts can be called plentiful. A tree that spawns many apples is plentiful.