In a nuclear reactor, what do control rods do?
Control rods absorb neutrons in the fission process to allow scientists to control the rate of the reaction.
Nuclear fission happens when neutrons, moving at just the right speed (neither to fast nor too slow) collide with and are absorbed by large nuclei, such as Uranium 235. The neutron is the most effective "bullet" because it lacks a charge, and will reach these nuclei without being repelled by electric force.
The process is a chain reaction in that the fission event produces three (I believe that is the number) neutrons for every one that causes a nucleus to fission. So, you can imagine that, if left unchecked, the number of available neutrons would quickly grow until the reaction ran out of control. This is what happens in an atomic bomb.
In order for scientists to control the process, they need to be able to remove many of the neutrons. This is the role of the control rods. Generally made of graphite, these rods can be inserted or extracted to absorb more or fewer of the neutrons, so that the rate of the reaction is maintained at the desired level.