Refers to the moment when dawn first breaks, and the sun begins to bathe the world in a beautiful, warm glow. It is often used in phrases such as:

"We'll get started at the crack of dawn", or "The rooster crows at the crack of dawn".

It's quite a nice term, under-used if I do say so myself, a rare quality, considering most every word I know is used and used and used, over and over and over again. (eg: over, and used, those words appear far too often..) :)