Okay kiddo, so you know how different people in different parts of America have different words for things? Like, in some places they say "pop" for soda, and in other places they say "soda" or "Coke" even if it's not actually Coca-Cola? That's because people in different places have grown up saying different words for things.
It's kind of like if you have a friend who uses a different word for "blanket" than you do. Maybe you say "blankie" and they say "snuggly" or something. It's just a different word for the same thing.
But sometimes the words people use for things in different parts of America can be really different! Like, in some parts of the country they call a sweet, soft drink "pop," but in other parts they call it "soda," "soda pop," "tonic," "soft drink," or even "co-cola" (which means Coca-Cola, but people use it to mean any kind of cola).
And it's not just drinks, either! People in different parts of America use different words for all kinds of things. For example, in some parts they call the night before Halloween "Mischief Night," but in other parts they call it "Devil's Night" or "Cabbage Night." And some people call a shopping cart a "buggy," while others call it a "trolley" or a "shopping caddy."
So basically, American English regional vocabulary just means that people in different parts of America have grown up using different words for things. It's like a big language quilt made up of all these different patches of words and phrases. And the cool thing is, even if we don't use the same words for things, we can all still understand each other, because we're all speaking the same language!