In a live sound system there is a loop from microphone through mixing console through power amplifier through loudspeakers back to the microphone.
It's a complete circle. The microphone will always pick up some sound from the speakers, which will be re-amplified and travel the loop again and again.
But this isn't feedback, or what we commonly call feedback, yet.
What is needed for howlround, which is a slightly better word, is for the 'loop gain' to be greater than 1 (or 0 dB, it means the same).
Suppose that the system is active yet completely silent. Now click your fingers in front of the microphone. The signal will pass through the system almost instantaneously.
Here comes an important point…
If the sound of the click from the speakers is louder at the microphone position than the original real-life finger click, then the loop gain is greater than 1.
If howlround hasn't happened yet, the finger click would probably have been enough to set it off.
But if the click from the speakers at the microphone position is quieter than the original click, then the loop gain is less than one.
There will be no howlround.
While it is true to say that when the loop gain is greater than 1, howlround is a near-certainty, it would not be equally true to say that a loop gain of less than 1 is entirely safe.
When the loop gain is just under 1, then you will not hear howlround but you will hear 'ringing'. Any input to the microphone causes the system to 'ring' at the frequency where the loop gain, taking into account the acoustics of the room, is highest.
Ringing is unpleasant and a wise sound operator will take suitable action to bring the loop gain well under 1.
Good advice… Have the microphone close to the sound source; point the loudspeakers away from the microphone and the microphone away from the speakers.
P.S. Phase is an issue, but not as big an issue as level because level is very much more controllable.