During a solar eclipse, the Moon blocks the Sun's light from reaching the Earth. What is the underlying reason for this phenomenon occurring?

The underlying reason is that the Moon’s position and size happen to match the Sun’s when the three bodies line up. Because the Moon’s orbit brings it directly between the Sun and Earth, its disk covers the Sun’s disk from our perspective. The Moon’s angular size in the sky is almost the same as the Sun’s, so it can block all of the Sun’s light in that small region. When the alignment is perfect, the Moon’s shadow falls on Earth and the Sun’s light is blocked, creating a solar eclipse. For example, on August 21, 2017, the Moon passed directly in front of the Sun and people along the path of totality saw the Sun disappear for a few minutes.