Here is some basics about nuclear physics, because this is basic I will only explain a bit about uranium.
Here is an illustration of uranium 235 getting into contact with a neutron, turning it into uranium 236 which is unstable and because of that there happens a nuclear fission that results in 2 smaller atoms and 2-3 neutrons being shot out, this also results in a lot of radiation and force. The neutrons being shot out can hit other Uranium 235 making a chain reaction.
In a nuclear reactor, you want to control the chain reaction so the reactor doesn’t blow up, to do this there is control rods that pick up some of the neutons being shot out so that the total amount of neutrons stays at a stable amount, avoiding there being to many enough for it to explode, while still being enough to keep the chain going.
If you are intrested in more you can visit wikipedia or watch this video on youtube
Nuclear physics for bored people 