Asteroid fields are large collections of drifting asteroids held in one place by gravity. Often they are the remains of a planet that has been destroyed, such as Alderaan or Byss. Sometimes either during or after their formation they are attracted by gravity to a nearby planet such as Jupiter or Geonosis and form rings around it, which viewed from a distance are very beautiful and inspiring but up close are just a bunch of rocks.
Travelers who are not robots, cyborgs or Corellians and cannot use the Force are advised to avoid asteroid fields at all costs, because it takes phenomenal reflexes and skill to navigate one.