During fusion, for instance hydrogen fuse to form a helium nucleus which has lower mass than the sum of its constituents.... Why is it so?

I have read that energy is required to break apart the nucleus to its nuclei hence energy is required but here energy is also required to bring the two nuclei together so why is energy released ? Also i have read that helium nucleus has more negative potential energy than hydrogen nuclei ..... Why ?

Please answer in detail with examples and no (if possible) complex mathematical terms.....