A solar eclipse occurs when the Moon moves in front of the Sun and hides at least some part of the Sun from the earth. In a partial eclipse, the Moon covers part of the Sun; in an annular eclipse, the Moon covers the center of the Sun, leaving a bright ring of light around the Moon in a total eclipse, the Sun is completely covered by the Moon.
It seems rather improbable that a celestial body the size of the Moon could completely block out the tremendously immense Sun, as happens during a total eclipse, but this is exactly what happens. Although the Moon is considerably smaller in size than the Sun, the Moon is able to cover the Sun because of their relative distances from Earth. A total eclipse can last up to 7 minutes, during which time the Moon's shadow moves across Earth at a rate of about 5 kilometers per second.