Oscilloscope Average Acquisition Mode
Start with Normal 
If you are only interested in DC voltage, you can go to Average to help reduce noise. But why use a 'scope? Use a multimeter instead - it is almost always more accurate.

Perhaps you are interested in noise pulses on that LM317 power supply, but the noise hides them, and you wish to use averaging to see those pulses more clearly.
 In this case, you must establish a reliable trigger. You must establish and verify that you have a reliable trigger in Normal mode.  If those noise pulses are related to the line frequency (50 Hz, or 60 Hz), then a line trigger is a good choice.

Otherwise, you might trigger from the signal source (Channel 1 or Channel 2 for example). And the trigger must be reliable before you go to averaging display. An untriggered source signal or improperly-triggered source signal gives an undulating and incorrect display in the Average mode.

It should be obvious that a single event (rather than a repeating waveform) cannot be averaged in Average mode.
The default whenever you probe something new, should be Normal.