They absorb neutrons and slows them down. If the control rods are pulled up again the fission process accelerates. The heat you get from the fission is lead away by a coolant, ordinarily gas or water. It passes the fuel pack and pick up the excess heat. It will now pass through a boiler or heat exchanger where it will deliver its heat to water which in turn will power the steam turbines.
All fuel needed for about a year is inserted into the reactor alla at once when it is started.
At the end of the year there is still a lot of fissionable material left, but not enough to keep the process running at a reasonable speed. Before the process stops completely, the used fuel pack is removed a new one is put in. In one year, only 1.5 tonnes of uranium-235 is used for a 1000 MW reactor. 1000 MW is roughly the energy demanded by a fairly large city. Fission products are collected inside the fuel rods and gets in the way of neutrons and dampens the process.