There's two key pieces of info on the screen when you sneak. One is the eye, which tells you whether something has spotted you (full open eye), is looking for you (half eye) or is unaware of you (closed eye or just a line). The other is the noise meter which fills up with the more noise you are making. Either one can make an enemy aware of you. If you get spotted you have to break line of sight and be quiet for long enough for the enemy to "lose" you.