Doesn't sound like you've read the rest the book, if you don't know what a Swift enumeration or struct is. "Library evolution mode" is a mode for the compiler that, if you read the rest of the book, you know enables ABI stability, which is what "@frozen" is all about. A "Nonfrozen" type is a type (protocol, struct, class, enumeration) that is not marked "@frozen". Consequently, a "frozen type" is a type that is marked "@frozen", which should be apparent from the context of the page.
This is not a standalone web page, but, one of several of a extremely well-written book, that has made been available as series of related web pages.
I am a very capable programmer, having spent 50 years in the business, and I have had no problem understanding this page whatsoever.
To the moderator: I'm sorry for the rant, but, it seems to me that we've been seeing more folks "demanding" that circumstances change because the state of being is "completely unacceptable." The tenor of the attitude seems to me to be completely att odds with the aims of these forums.