[Wavytone]

To go critical the earliest reactors were based on a design in which enough fuel was packed in a given volume to exceed the threshold at which it will go critical, ie if unchecked a chain reaction would start spontaneously the moment the fuel exceed the threshold. The fuel was a static "pile" of material. To stop it literally melting while being assembled, a moderator medium was added which absorbs or slows neutrons to the point they cannot cause a reaction - depending on the type of reactor the moderating material included heavy water and/or rods of boron

There are two ways to increase the reaction rate - either:

(a) push more fuel into the heart of the reactor
(b) remove some of the moderating material.

This causes the reaction rate to increase and if unchecked it will melt the fuel, possibly igniting it, and possibly parts of the reactor vessel holding it. To stop that, either

(a) the fuel rods are withdrawn, or
(b) moderating rods are pushed in.

For these reasons fuel in modern reactors is packed as rods than can be pushed in or out, and the moderator includes a series of rods that can also be moved in or out.

For safety the reactor designs are supposed to include included a fail-safe arrangement such that if it overheated, moderator rods would fall into the reactor (using gravity) and some of the fuel drops out the bottom. If the fuel rods ignite the result is an assortment of highly radioactive powdery oxides (from the various isotopes present) - hence the need to encapsulate the Chernobyl reactor in a concrete sarcophagus to stop the stuff escaping into the environment. Evidently the russian design wasn't fail-safe.