It takes energy to fuse atoms together, but the act of fusing atoms together also releases energy, aka, you have to break a few eggs to make an omelette.

As long as the energy coming out of fusion is higher than the energy needed to do it, a star can exist happily.

For all the elements before iron, this is the case, more energy comes out.

Iron, however, is the first element that takes MORE energy to fuse than it gives back, the star isn't so happy any more. It now has to use a lot of eggs to make a rather sh***y omelette.