Asteroid fields are regions of space occupied by numerous irregular shaped bodies. Asteroids are the most common celestial mass found within these fields, but dwarf planets such as Ceres or Pluto can be found within them as well. Asteroid fields that circle around a solar system are often referred to as asteroid belts.
Asteroid fields are believed to be the remnants of a proto-planetary disc, once present in a solar system's early formation. Asteroid fields are areas that were never cleared out by larger planets.
The density of the asteroids makes collisions frequent, but only on an astronomical scale. In reality, asteroids are actually quite far apart. Contrary to depictions in Hollywood, an accidental collision by a probe or a starship is extremely unlikely.
Asteroids carry minerals such as iron, nickel, titanium and even water. This makes asteroid fields a valuable location to exploit by spacefaring civilizations, as they are easier to claim and inhabit than alien planets which may be inhospitable. To mine an asteroid, one must either capture a small one or create a settlement on its surface.