Why does meteor shower occur?

Comets have an elongated ellipse path around the Sun. Because of their loose consistency, when passing close to the sun they form debris which makes the tail of a comet - small particles following it on its path.

The Earth does not have a strictly circle path around the Sun, its path is slightly ellipse and in some places it overlaps with the path of some comets. When the Earth intersects these places along its annual trip it sometimes runs into decomposed particles of the comet - the debris, and when these particles enter the atmosphere they burn up and cause what we call a shower of meteors.