Foreword as an introduction by someone else than the writer talking about it.
I never understood the concept, maybe if I reread something I'd care what another person has to say about the book before I start, or if it appeared in the end discussing the book it will be an interesting addition. But as an introduction it spoils parts of the book and will probably bias the reader to interpret the book in a certain way.
The only exception is translators notes since this is one point where language barriers are important to know to understand.
Eh, it depends. I like it if the foreword has something interesting. Sometimes it's one written by an expert on that specific topic/author/genre, and they can have something insightful to say. Lin Carter would often have good introductions when he was the editor for the Ballantine Adult Fantasy series of books that revived pre-LoTR fantasy in the '70s. He was one of the first proper scholarly types to give fantasy proper attention as a genre.
The other type of foreword I like is when it's from an author's peers. I've read forewords from the likes of Asimov, Robert Silverberg, Ursula le Guin, J. G. Ballard, and other SF-F writers in which they're using that foreword space to talk about the book's author/work and their own experiences and perspectives of being someone from the same era/movement (New Wave, Space Age, Fantasy Boom, etc.) or from personal friendships (Asimov often wrote about his relationships in a jovial and affable manner).
I don't like the Neil Gaiman thing where he spoils the book he's writing a foreword to and then goes "hey that's where I got XYZ for in my stuff".
Another underappreciated related thing is a good afterword. One of the decent ones I've read was GRRM's afterword for a Roger Zelazny book, in which he passionately and tenderly reminisces about Roger Zelazny being a wonderful friend and mentor. It's probably why I can't really bring myself to hate on GRRM too much, because he just seems to be a genuinely sad and lonely old fella that misses his friends and loved ones. Every damned time I read him reminiscing on the past, I just feel bad for the fella.
A lot of them are the author covering their bases/trying to prevent Death of the Author interpretations of their work, which I find just pre-biases me in the opposite direction when reading it. If I like a book enough to re-read it, that's when I'll see what the author had to say in a foreword. In that regard the foreword Tolkien added to the 2nd edition of the Lord of the Rings is hilarious, since it's 5 pages of him saying "I started writing parts of this story in the fucking 1920's, it's not about WWII, stop saying it is you weirdos"
This is funny. I recall a plenty of writers who were around in the '20s-50s eventually writing forewords for editions of their works put out during the mass market paperback book of the '50s onwards. There's quite a few mass markets with "foreword by author" in them for a book that was supposedly originally put out like a decade or two before.
