Many state boundaries in the USA are along lines of longitude and latitude (meridians and parallels) and Four Corners is where the boundaries of Arizona, Colorado, New Mexico and Utah meet at a single point at right angles to each other, so it is possible for the visitor to have one limb in each of the four states.
It is the only place in the USA where four states meet, hence its name.
The monument is maintained by the Navajo Nation as a tourist attraction.