If the Earth and the Moon moved in one plane, each new moon the Moon really would have been exactly on the straight line connecting the Earth and the Sun, and would occur Eclipse. But the plane of rotation of the Earth around the Sun and the moon around the Earth do not coincide. So often during the new moon the Moon passes either above the Sun, or below it.
The visible path of the moon and the Sun in the sky intersect at two opposite points, which are called the nodes of the lunar orbit. Only when the new moon occurs near the site, it is accompanied by an Eclipse of the Sun. Such coincidences occur infrequently and irregularly. On average, a solar Eclipse happens 2 times a year.
During a solar Eclipse on Earth runs a small round dark spot Moonshadow, barely able to close a major city. Therefore, a full Eclipse of the Sun in the same place on Earth you can see every 200-300 years.
A total solar Eclipse. View from space.
(August 11, 1999, the space station "MIR")
See also: What is a solar Eclipse?