You could be referring to the concept of a time machine, which is, like YellowOC1 defined for you.
You could be referring to the H. G. Wells novel published in 1895: http://www.google.com/url?sa=t&rct=j&q=&am...
Or, what I think you are referring to is the Mac application that is used to back up your computer. If you use it often, you can backup the files on your computer with this program. If you accidentally delete something important, the Time Machine will allow you to go back in time on your computer to the point when that file wasn't deleted, but you will lose any new data that you added to your hard drive since then.
Hope this helps!