Kessel was a planet that hosted major mining facilities. The Galactic Empire operated a mining operation, employing numerous slaves, including Wookiees, to harvest spice. Slaves who were sent to Kessel worked in harsh, brutal conditions.
The planet Kessel was first mentioned in the 1977 film Star Wars: Episode IV A New Hope and the 2015 film Star Wars Episode IIV: The Force Awakens. It's first canonical appearance was in Star Wars Rebels: Spark of Rebellion, the premiere of the animated television series Star Wars Rebels.
The series used designs and concepts created for Kessel by Star Wars creator George Lucas, who developed its appearance, culture, and economy.