Please refer to this wiki-page, which says it best: http://wiki.blender.org/index.php/Manual/Ipo_Curves_and_Keyframes#Special_Notes_on_the_Time_Curve
"With the Time curve you can manipulate the animation time of objects without changing the animation or the other IPOs. In fact, it changes the mapping of animation time to global animation time (Linear time IPO). The Time curve is a channel in the Object IPO.
“In frames where the slope of the Time curve is positive, your object will advance in its animation. The speed depends on the value of the slope. A slope bigger than 1 will animate faster than the base animation. A slope smaller than 1 will animate slower. A slope of 1 means no change in the animation, negative power slopes allow you to reverse the animation. The Time curve is especially interesting for particle systems, allowing you to ‘freeze’ the particles or to animate particles absorbed by an object instead of emitted. Other possibilities are to make a time lapse or slow motion animation.”
In short: “?sdrawkcab nur ot emit rof ekil uoy dluoW” (Read the sentence backwards…)
Or maybe, “first forward dna neht sdrawcab and then forwards again?”
So our poor spacecraft is just accelerating out of the dock when it runs into a pesky time-warp field … suddenly “time” is running sdrawkcab (that’s the “backwards”… backwards), but the object is still gaining “speed.” Obviously the visual-effect in this case will be the same, but the fundamental reason is different. “Speed” is always “forward in time.” “Time,” of course, is “time itself.” When someone says that you can IPO everything in Blender … they mean it.
The Time IPO is most-prominent in the “Absolute Shape Keys” (RVKs) screen, where it usually shows up as a line with 45-degree constant slope. (The position of each horizontal line, representing each progressive shape, is determined by where the line would intersect the Time IPO at the current frame.) This IPO curve (shape) therefore represents the normal, constant, forward progression of Time. But… time does not have to be linear and it does not have to go forward.
Time is important when you consider “actions.” You want to effect a certain action, already defined, but you want it to slo-mo or to run backwards. The Time IPO is what makes that sort of thing possible.