The asteroid belt is a torus-shaped region in the Solar System located between the orbits of Mars and Jupiter. It contains a vast number of solid, irregularly shaped bodies called asteroids or minor planets, which vary in size from tiny rocks to dwarf planets like Ceres. The total mass of the asteroid belt is small, estimated to be only about 3% of the Moon's mass, with the four largest asteroids—Ceres, Vesta, Pallas, and Hygiea—making up the majority of the belt's mass.
The reason the asteroid belt exists is that during the early formation of the Solar System, the strong gravitational influence of Jupiter prevented the material in this region from coalescing into a larger planet. Jupiter's gravity caused the planetesimals to collide and break apart rather than form a planet, leading to the formation of this belt of smaller bodies instead of a single planet. This region represents leftover debris from the solar system's formation that was gravitationally disrupted by Jupiter.
Thus, the asteroid belt is essentially composed of remnants of the early solar system that never came together to form a planet because of Jupiter's disruptive presence. It acts as a boundary between the inner rocky planets and the outer gas giants in our solar system.