The usual oversimplified image of forces inside a nucleus is to picture a hill made out of foam rubber. Place some heavy stuff (nucleons) onto the top and they make a dimple* at the top which holds them and keeps them from rolling away.
While the protons would love to separate and roll down the hill, the dimple holds them in. Assuming that the forces (ratio of protons-neutrons) and other stuff** work out right. Else you get decay or fission.
Note that rolling nuclleons up the hill and over the edge takes energy. Plus pushing the dimple down takes energy. It’s springy. You get the energy back during decay or fusion.
Well, okay. Not always. Curve of binding energy and all that. Like I said: oversimplified. You can try to save things for “light” elements by talking about the bottom of the dimple being lower than the bottom of the hill, so for light elements you get net energy by rolling stuff over the lip of the dimple and down into the deep dimple. And at some point around here the analogy starts to get stupid.
- Like a crater, only cuter.
** Nuclei love having even numbers of things.