Each line in the window is referred to as a “time entry.”  The time entry with the italicized text is the active time entry, or the time entry that is tracking time.  In the example document above, the active time entry is also selected (highlited).  To stop or pause an active time entry, click on it to select it.  Then you'll be able to click on the Stop or Pause button.  Otherwise, if you have multiple time entries or no time entries selected, some buttons may be disabled.

Click on a time entry to select it.  To unselect it, hold down the shift key when clicking on a time entry.  Unselect all time entries by selecting “Unselect All” from the Edit menu, or scrolling to the end of the list and click just below the last time entry, or click on all time entries while holding down the shift key.

Each time you click on the Start button, a new time entry is created and appended to the time entry list.  If the time becomes greater than 99:59:59, it rolls over to 00:00:00.