It happens that the sun and moon each, by coincidence of size and distance, take up the same area of the sky. The moon, the smaller of the two, is much closer than the sun to the Earth. So, when the three bodies align, with the moon between the Earth and the sun, it is capable of blocking the solar disk, and doing so rather exactly.
The moon travels around the Earth in just under a month. So, one might expect a solar eclipse monthly. But that is not the case. The reason is the moon does not go around the equator, rather it has an orbit that is slightly inclined to the plane of the equator. Then, the Earth rotates on a tilted axis. So, the moon misses being in the ecliptic position most of the time, and only occasionally is in just the right place to cause an eclipse of the sun.
Simply put, the Earth passes through the moon’s shadow. But, not all of the Earth is involved in a solar eclipse. The moon is hundreds of thousands of miles away, and the sun is much larger than the moon. So, the shadow is a cone, and is rather small when it gets to the Earth.
As an analogy, imagine having a large light, and a small object directly in the path of the light. The shadow right behind the small object is obvious, but as the light from the sides of the large light source pass the smaller object, there is some light that is converging behind it from the outer parts of the light source. The shadow gets narrower the farther away from the light source the light travels, until there is convergence of light from both sides, and the shadow disappears altogether.
People in the shadow are said to be in the umbra, the area where all of the light from the sun is blocked. Think of the word umbrella, where rain is blocked. In the umbra the sky would be as dark as night, and stars would be visible.
People close enough to the umbra would be in the penumbra, an area where only a part of the sun is blocked, and see the solar disk with a piece missing.