Cosmic rays are high-energy particles that travel through space and reach the Earth's atmosphere. They primarily originate from sources like supernovae and active galactic nuclei. When these particles collide with atoms in the atmosphere, they create a shower of secondary particles, which can be detected on the ground. These rays consist mainly of protons, but they can also include heavier atomic nuclei and electrons. Although cosmic rays are constantly bombarding the Earth, the atmosphere and magnetic field provide protection, preventing most of them from reaching the surface.