An adjustment in basic terms refers to the procedure in which the chiropractor gently and skillfully applies a controlled force from his or her hands to a specific joint in the patients spine.
Adjustments for the most part should not hurt. Chiropractors train for years to perform these correctly on patients.
There is an associated "pop" sound which is simply a gaseous reaction within the joint of the spine.
What the chiropractor is doing is getting the joints of the spine to move correctly and restore the spine to as normal a state as possible.