To deorbit something is to cause it to leave an orbit around a celestial body. Most commonly, the word is used as a verb to describe an action or event which causes an object to leave a stable orbit and thence to descend to the surface of the body it is orbiting. It can also be used as an adjective as in the term deorbit burn. Satellites and spacecraft, as well as debris such as expended boosters are all typically deorbited if possible so as to prevent them from becoming uncontrolled space junk. They may also be deorbited deliberately in order to ensure that their re-entry and descent is a controlled one rather than a random one at an unknown later time - in this manner, they can be directed to re-enter Earth's atmosphere over uninhabited areas to minimize the chance of injury or damage from debris. It does not necessarily refer to objects in Earth orbit; a spacecraft could be deorbited onto Mars, for example. This would be distinct from a spacecraft which entered the Martian atmosphere without first entering a stable orbit, however.
Although it is not often used in this manner, it can technically refer to the act of removing something from a stable orbit into an escape trajectory.