(32/36) Intro to CSS3 Animation (part 2) - @keyframe Rules
The CSS3 keyframe rules specific what will happen at certain points in your keyframe animation. The percentage values indicate at what point in the animation something will happen. Within the percentage you can add CSS values specifying what will happen like changing the elements positioning, z-index, etc.
For more information refer to the W3C spec page: http://www.w3.org/TR/css3-animations/