Natural units are a system of measurement in physics where certain fundamental constants are set to one. This simplifies equations and calculations by eliminating the need for conversion factors. For example, in natural units, the speed of light, c, and the reduced Planck constant, ħ, are both equal to one, making it easier to express relationships between energy, mass, and time. These units are particularly useful in theoretical physics, especially in areas like quantum mechanics and general relativity. By using natural units, physicists can focus on the underlying principles without getting bogged down by conventional units like meters or seconds.