It happens right in the middle of a book you're enjoying: the story stops while the author tells you everything he or she knows about ark-building, quantum physics, or blind cross-stitching. If you're really into that particular subject, you nestle into your chair and think, "Cool!" If you're not, you let your eyes roam ahead to the spot where we get back to the story. We've all had the experience of reading a book where everything stops while two characters sit down and discuss some topic at length while we wonder, "What about that gang of crazed assassins breathing down your necks?" Even worse is when this overlong discussion has nothing whatsoever to do with the situation that faces them.
It's a strong temptation for authors. We research and learn things that intrigue us, and we want to share them with others. Or we know things from our "real" life that we long to...well, show off to the world. The question is, does the world care?
I think the answer is yes if you're careful. Information that is judiciously slipped into a narrative makes it interesting, and reading is the way we learn about all the things we can't actually do. Most of us enjoy reading passages about things we have experienced as well, so we can compare our opinions and memories to theirs.
The problem comes when the information is so off-topic or so extended that it takes the reader out of the story. He or she finds himself thinking, "This has gone on for some time now," instead of focusing on the story's events. Anything that does that is probably dumping, and some editor somewhere should have suggested that the author cut parts or weave them into the story better.
Often a particular writer has prejudices that show up over and over in his writing. Again, if these opinions are introduced subtly, they add to the reader's enjoyment. But sometimes as I read I find myself thinking, "Here he goes again with the diatribe against lawn bowling." Simply putting your opinions into a character's mouth doesn't hide the fact that they're your opinions.
I'll be the first to admit, it's hard to keep out of your writing the things that fascinate you. History is my passion, and I'd love to tell you all about Henry VIII and his wives, but I know most people don't care quite as much about that as I do. (Thirty years of teaching convinced me of that.) So when I edit, I make myself leave out a lot of "background" that I'd like to include. It's hard to do, but in the end it assures that I have a novel, not a textbook.