An eclipse of the Sun occurs when the Earth passes through the Moon's shadow. A total eclipse of the Sun takes place only during a new moon, when the Moon is directly between the Sun and the Earth.
When a total eclipse does occur, the Moon's shadow covers only a small portion of the Earth, where the eclipse is visible. As the Moon moves in its orbit, the position of the shadow changes, so total solar eclipses usually only last a minute or two in a given location.