Mass is made up of atoms that have neutrons, protons and electrons. The elements all have a specific number of protons when you add protons it is fusion when you remove them it is fission.
The protons have an amount of energy that holds them together, while they are together this energy is stored as mass. Adding protons to anything with less protons than iron means it needs less energy to hold together so it releases what it was storing. If you remove protons from anything above iron it also releases the stored energy.
Iron for some reason has the highest binding energy so once you are there you need to put energy in to add or remove protons.
PS. This is why when spider-man drags the mini Sun away from the iron and dumps it in the lake I scream at the tv
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