Light is made of photons, which are fundamental particles. Because photons have no mass, relativistic effects allow them to travel at appropriately the speed of light.
Light and photons played a key role in physics through the years. While light behaves like a particle in some ways, it also displays wave-like properties as well. Eventually, experimental research demonstrated light's dual nature, so photons are sometimes called wave particles. Photons played an important role in the development of quantum mechanics. The double-slit experiment demonstrated that phenomena previously believed to be fully deterministic had probabilistic properties and helped provide experimental evidence for early quantum mechanical theories.