These were the technical terms popular with the people who were studying language (or, to give them their fancy titles, semioticians). A sign is something that stands for something else, like the word "cat." And the "signified" is the thing the sign is pointing at—in this case, an actual cat.
Readerly texts are easy on the reader. They're all about telling the story in order and making it clear what we're supposed to get out of it. Kind of like Little Red Riding Hood: everything proceeds in logical order, and we've got this super clear moral—watch out for wolves that can talk and dress up like grandmothers. Actually, maybe just avoid wolves altogether.
Writerly texts are just the opposite of readerly texts. Bottom line: they're hard to read. Everything might be out of order, or there might be a whole wagon full of different ways to read the story (the equivalent of a Choose Your Own Adventure). I'm totally into these texts: they make readers more like writers, since they have to take an active role in putting the story together for themselves.
I say that our idea of the author is too wrapped up with genius and originality. This whole author-role isn't possible any more anyway. Today we have "scriptors" (or writers). That's right, author: you're dead.
The scriptor (or writer) is like the modern day, fallen version of the author. Writers aren't the same mythical geniuses as authors were in ye olden days. Authors used to create from scratch, but now writers just put old material together. La-zy.
Ever notice how texts include details that don't seem to have any purpose? Well, those details are there to make the fictional world feel more real.
Don't you think a lot of our culture is about giving us certain ideas or "myths" about the world? These myths are like stories that we tell ourselves so often we think they're the objective truth about things. Take milk for an example: do we really see milk as just milk? Or is it all the stuff (the myths) we've learned? When you think of milk, you might think of happy cows out at pasture, family dinners, cookies and milk after school—and how milk is nutritious, wholesome, and pure. All myths, I say!