When I was editing books on a daily basis, I saw a lot of repetition with individual words and phrases and also with concepts of basic storytelling, plot points, and ideas. I would highlight the repetition for the author and suggest they remove it, since they'd already said it earlier in the book and, in case they hadn't noticed, I wasn't an idiot.
The response I received usually looked something like this:
The repetition was intentional for effect.
This annoyed me to no end, because although they obviously thought their repetition was for effect, it was more often than not having the exact opposite effect on me. Namely, boredom. What I learned while editing was that it's actually not hard to write a long book. Just repeat yourself over and over and over again, and all of the sudden you're at 200+ pages and pretty proud of yourself.
If you're willing to let an editor who knows anything take a look at it, they will cut your page count nearly in half, and there begins the real work of writing: choosing words, phrases, plot points, and characters well enough that you aren't repeating yourself unnecessarily. It gets harder to add pages when you're looking closely at repetition and flab. (On a personal note, this is often where I get trapped and a large part of the reason why I've never written a story longer than 25 pages.)
Repetition can be as obvious as using the same adjective to describe a noun (ie. saying "the black car" every single time) or as subtle as repeating the same tone or moral from character to character or even book to book (ie. creating each character in your book with the same flaw or constantly putting them in the same situations).
There are exceptions to all of this, of course, because sometimes repetition is necessary. For example, say you have a character who's playing poker, and the main character realizes that every time that particular character winks his left eye, he's bluffing. Having a character wink his left eye a time or two during a poker game is necessary to prove a point. Just make sure you aren't copy/pasting the exact same lines or using the same words to describe the scene. There's a difference between necessary character development and unnecessary repetition.
You have to create new characters and scenes and responses for readers to feel engaged. If every single scene has the same conversation, it gets boring fast. And if every single car is black and every single character's teeth are white, no one and nothing will stand out.
Sometimes repetition is easy to spot, and sometimes it's more difficult. The first thing to do is put your writing away for a day or two--even a week or two--and when you come back to it, you will be more able to notice repetition both large and small. The next thing you need to do is give it to someone else to read. You would be amazed how much good a fresh eye can have on your work.
I hope that's helped you understand the basics of repetition. I know that was a short overview, so let me know if there's anything I need to clarify. And if you're interested, check out my post on removing the flab for a more in-depth essay on trimming out unnecessary words and phrases (like really and other such adverbs).
(Also, is it bad that I'm glad there are only 2 of these writing posts left? I feel like I'm losing steam...
Maybe it's because I changed jobs and no longer edit every day. Now I write every day, and I never feel like writing about writing. Sometimes I don't feel like writing at all. But that's a post for another day.)