How and why does the eclipse of moon occurs?

The moon orbits the Earth. And, it's orbital plane is off from the Earth's orbital plane around the sun by about 5.5 degrees.
When the moon is full, and within about 2 degrees of the nodes of these two planes, then it can cross into Earth's shadow - and, that's an eclipse. So, when the Sun, Earth, and the moon are lined up, a lunar eclipse can occur.
When the moon is full, but not close to either node of the two orbital planes, then the Earth's shadow is either above or below the moon, and no eclipse occurs.

When the Moon passes through the Earth's shadow.

The moon passes through the shadow the Earth casts into space.
https://en.wikipedia.org/wiki/Lunar_eclipse

Basically, it happens when the Moon's orbit puts the Earth directly in line with the Sun.
It happens when it coincides with the node that crosses at 0 degrees in its 5 degree plane..
It usually happens about once or twice a year. and is pretty spectacular.

When the Moon is behind the Earth.

The earth gets between the sun and the moon. (Note that this can only happen during a full moon.) The earth then blocks the suns rays from illuminating the moon.

Solar eclipses are relatively rare; they happen when the Moon moves directly and precisely in front of the Sun. Why are they so infrequent, and why do solar eclipses always come in pairs with lunar eclipses?

It is all down to a mix of orbital alignments and cosmic coincidences. The Moon is able to cover the Sun’s disc precisely because the Sun is about 400 times bigger than the Moon, and also about 400 times further away, meaning they have the same size in our sky. That’s a huge coincidence! With all that sky above our heads though, what causes the Sun and the Moon to be in the same position in the sky at the same time?

It’s all a question of orbits. Earth orbits the Sun along a plane that we call the ecliptic – all of the planets orbit along this plane, more or less. The Moon, however, orbits Earth at an angle that is tilted to the ecliptic, by about five degrees. This means that on each orbit around Earth (a lunar orbit lasts 27 days), the Moon only crosses the ecliptic at two locations and these are the only two opportunities for an eclipse, be it a solar or a lunar eclipse.


Now, an eclipse of either variety can only happen when the Sun, Moon and Earth are in a line. Usually, the Moon crosses the ecliptic when they are not in a line, so we do not see an eclipse. However, on odd occasions – roughly once every 18 months on average – the Moon crosses the ecliptic at a time when it is aligned with Earth and the Sun. If this crossing takes place when the Moon is between Earth and the Sun, we see a solar eclipse. Two weeks later the Moon moves to the other side of Earth, with the alignment still the same, causing the Moon to slip into Earth’s shadow. Here on Earth, we see a lunar eclipse.

So solar and lunar eclipses always come in pairs. This year, following the dramatic total solar eclipse on 20 March, a total lunar eclipse will occur on the night of 4/5 April, which will be visible from the Southern Hemisphere, in particular Australia and New Zealan

youtube.com. search. "lunar eclipse".