Can someone explain Pokemon to this grandma?

My 3 year old grandson told me he wanted a Pokemon car for his birthday.
I couldn’t find a car locally, so I ordered one online (it will be late) and got him some cards and a stuffed Pokemon.
His mom doesn’t even know where he heard of them (maybe pre school?) but I would like to understand what you’re supposed to do with them or what the story is so I can understand and play with my grandson.
Can someone give me a clue?