INTERVAL is how often, in milliseconds, the timer event will fire.
The code that fires is that which resides in the timer event of the timer.
TIME() pulls the current time and TRANSFORM(TIME()) turns the time into a character string, which is accepted by label captions.
thisform.label1.Caption=TRANSFORM(TIME()) assigns the caption of the label dynamically, and since it is in the timer event of the timer, is re-set every 1000 milliseconds (i.e. second).
You can add day etc too:
thisform.label1.Caption=CDOW(DATE())+" "+TRANSFORM(TIME())