It just means that each track point now that Maemo Mapper keeps track of now also has a timestamp with it. So now, when you Save a track, Maemo Mapper not only saves the coordinate data, but also the time at which you were at each coordinate.

Most commonly, it is used to "play back" a trip using special replay software. It can also be used to estimate average speed or total trip time (or time between any two points, etc.).