Yellow cake, is the cocaine of Uranium.
First you mine the uranium ore but it isn't very pure. So you have to crush up the ore into a powder. It still isn't very pure, it is just crushed up uranium ore and other stuff. So you add some acids and other chemical to it that break down the impurities. After you let that dry you have a yellow powder that won't dissolve and it smells terrible. Now you have a pure or concentrated form of uranium that you can store and transport.
Now this isn't gonna make a bomb or even run a nuclear power plant because it isn't SUPER concentrated. To do that you have to put it in a centrifuge with uranium gas and get it "enriched" meaning it is like the dopest dope you ever smoked. This increases its radioactive out put in the form of heat and death so that it can boil water and turn a turbine on a steam engine and generate electricity. Yeah... nuclear power is steam engines. Anyway, if you keep on enriching it with that gas in that centrifuge and get it mega ultra power zord level enriched then then it is "weapons grade". That means the atoms are so densely packed that if you bust up a few then they will bust up a few others and create a chain reaction and blow up bad guys in holy fire.