A total solar eclipse takes place when the moon comes between the sun and Earth, and castes its umbra, the innermost and darkest part of its shadow, on Earth completely blocking the sun's light from view. The sun is about 400 times larger than the moon but also about 400 times farther away, so it appears the same size as the moon. So since Earth is about 3.7 times wider than the moon and the shadow created by the moon is conical in shape, narrowing at Earth, only a narrow path across Earth's surface will experience a total eclipse. In fact, if the darkest part of the moon's shadow ended before reaching the distance to Earth's surface, we would not experience a total eclipse at all. It would be an annular eclipse, where a bright ring of the sun would be visible around the moon's shadow. This is called the antumbra, beyond the umbra.

The eclipse happens because the moon casts a shadow on the earth. The moon is also smaller than the earth, so the shadow of the moon can't cover the whole earth. On top of that, the size of the moon in the sky is only slightly larger than the size of the sun in the sky, which means that the area that is in total shadow (i.e. total eclipse) is much smaller even than the size of the moon. Combine all of these things together, and the path of totality is only a couple of hundred kilometers across.

If you want to see how this works, try doing experiments using a light bulb (the sun), any round, opaque object like a tennis ball or something (the moon), and a globe or map of the earth.

It's pretty rare for the sun and moon to line up with the earth, and so when you're too far to the north or south of the total eclipse, the sun and moon aren't totally lined up.

A solar eclipse will only happen when the sun moon and Earth are in line. It happens when the moon blocks the light from the sun. There are some facts that make such event rare: Earth is orbiting around Sun, the moon is also orbiting around the earth (both orbitals are elliptical), and the earth is rotating around its tilted axis.

To have the total solar eclipse, the moon must be close enough to Earth such that the moon appears larger or equal to the apparent size of Sun during the eclipse. Also only those in the umbra (the shadow because of the moon blocking the light from the sun) on Earth can access the total solar eclipse. Since the sun is much larger than the moon, the region on Earth in the umbra is very small. That is why only some states will have the total eclipse while the others not.