The min/max-width media query doesn't make grammatical sense

The min/max-width media query doesn't make grammatical sense

I'm finding the concept of the (min-width/max-width) media query a bit
confusing.
Naturally if I was to design a media query I would want to say (in
pseudo-code)....
if(screen.width < 420)
{
ApplyStyle();
}
This concept of talking about min and max doesn't make any sense since the
'min-width' of something like a div element is a command not a question.
I know that the following is true when my screen goes below 420px...
@media screen and (max-width:420px) {
}
I just don't know why because the max width is something I tell it to
have. If I have told it to have something why is css checking it? Surely
it already knows it.
I'm perhaps missing the grammer/context here. Can someone please explain?