How do you use the new type checker?

So not recently but roblox posted Luau Recap: May 2020 and i found some new type checker there, i had no idea it even existed so i tried using it and i have no idea how it works, i've red the whole post but did not understand even single thing from it, i then tried to use print tactic but got no result, could anyone please explain me this

local n = 50

type n = (string) -> boolean -- ?what??

what are these -> ? and how does the type thing works?

Thanks, kirda