Answer:
To absorb neutrons before the neutrons can initiate more fusion reactions.
Explanation:
Control rods, usually made of compounds like graphite, absorb neutrons and prevent them from going everywhere and bombarding other molecules of the radioactive compound being used, usually U-238, and prevent a chain reaction from starting. When U-238 is bombarded with a neutron, it releases 3 others, which hit 3 molecules of U-238, releasing 27 neutrons in total, and so on. This must be controlled, otherwise an uncontrolled chain reaction could easily become highly explosive (Nuclear bombs that utilise U-238 use this principle to cause a chain reaction at a larger scale that releases huge amounts of energy that cause the initial explosion when the atom bomb is detonated).
