great question, which takes me back to my physics classes in secondary school.
Basically, nuclear fission is the slow breakdown of radioactive elements, from one large atom into two smaller atoms. There is a tiny loss of material from this, where matter is converted into pure energy. This gives off a lot of energy, which we use in standard nuclear fission powerplants. In a nuclear bomb, two lumps of radioactive material are shot at each other at great speed with smaller explosions. This initial energy causes a chain reaction, converting all these atoms into two smaller atoms rapidly, giving off huge amounts of energy. Again, these new atoms are not destroyed but converted into two radioactive atoms. A bomb spreads this material over a huge area, which continues to break down slowly into something more stable.
Fusion, is when energy is released when two smaller atoms are combined to create a larger atom. It basically squeezes two hydrogen atoms to create helium atom and lots of energy. Nothing is harmfully radioactive in this , but it releases a HUGE amount of energy. So when a hydrogen bomb explodes, there is no “nuclear fallout” because there are no radioactive atoms created. Its the same with power plants. Though we have not perfected fusion energy yet(we cannot stop the chain reaction from causing an explosion!), it would be much cleaner than regular fission nuclear energy as no radioactive material is required or created.
Did you know that the Sun is one MASSIVE fusion powerplant?! That’s how it can release so much energy!