What is "plastic #1 - 7"?

Still fine tuning my recycling. On my guide sheet it says they accept “plastic #1— #7 for recycling.”
I’m trying to find out what that means. I find the term “plastic #1 – #7” listed when I google, but no explanation. It’s like we’re just supposed to know what it is.
I assume it’s some sort of weight thing. For example, they don’t take plastic bags so maybe that’s a plastic type that’s above a 7? Where do plastic fountain cups come in? Straws? Fountain cup lids?
I know they don’t take lids from, for example, those asshole individual water bottles people buy, and I think it’s because they bung up the works during the sorting. Or is it because of the kind of plastic that it is?

I called the recycle place and the receptionist said she didn’t know either.

Fluther knows, I know she knows!