Time registration can be seen as the traces your clock makes during the day; each time you start or stop working on an activity, the clock will leave a footprint. These footprints are saved, so all sorts of analysis can be performed on them at a later stage.
A single time trace consists of a start- and a stop time and the activity that the time period is registered for. When starting a new time trace, you select the activity you will start working on, and then you just press the start button. Once you are done working, you press the stop button and the new time trace will be added to the list of footprints.