When the Moon’s orbit is closest to Earth at the same time the Moon is full, then a supermoon occurs.
A Supermoon requires Two crucial conditions to occur
In its 27-day orbit, the moon must be at its closest approach, or perigee, to the Earth.
(1)
The moon must also be in its full phase, which happens every 29.5 days when the sun completely illuminates the moon.
(2)
Because the moon's orbit changes orientation while the Earth orbits the sun, supermoons only occur a few times a year (at most). For this reason, you don't see a supermoon every month.
When a full moon appears at perigee it is slightly larger and brighter than the usual full moon – and that's where a "supermoon" occurs.
We know that the Moon revolves around Earth in an elliptical orbit with a mean eccentricity of 0.0549.
So, the Moon’s distance from Earth (center-to-center) varies with mean values of 405,504 km at apogee (most distant) to 363,396 km at perigee (closest).
When the Full Moon phase occurs near perigee, the Moon subtends its largest apparent diameter as seen from Earth. This phenomenon is commonly known as a supermoon.