They say that every atom in our body was once part of a star?
What do you suppose single cells are composed of -- molecules, which themselves can be disassembled into atoms. Atoms of hydrogen eons ago were part of early stars which began fusing hydrogen into helium as part of their normal process. As stars ran out of their simpler component, they began fusing the helium into heavier elements. More massive stars had more powerful forces of gravitational compression and forced fusion to higher levels. Once the fusion process reaches the production of iron, the fusion process stops putting out heat which fought to resist the forces of gravitational compression. At that moment (or instant), the star would have undergone a supernova explosion. This shock wave is so powerful that it is capable of pushing fusion to heavier materials such as gold, silver, platinum, etc. All elements from helium on, including carbon, nitrogen, oxygen, phosphorus, sulfur, and so on, were created by early stars which lived and died before the formation of our Sun. We are created from this star stuff.
No one is necessarily saying that. But it is highly likely. What is certain is that everything apart from hydrogen and helium came from the interior of stars. And that particularly applies to everything with an atomic number higher than iron. The hydrogen in our bodies MAY have come from stars, it equally might have been part of the original cloud of material that condensed to form the stars of our galaxy and it might never have been part of a star. We all started from one cell. The atoms comprising that cell came from stars, as did the atoms in all our cells, as did the atoms that comprise our entire planet. We were once part of the earth. Before that our atoms were part of some star, or likely came from several stars. We earthlings are travelers in space.
Not every single atom, as it happens. At least 10% of our body is hydrogen, which may never have been part of any star. The answer is that cells don't create matter from nothing, but use the matter around them, and that matter was part of star because this planet formed from a cloud of gas and dust which was blown out of an exploding star. The cells of your body just have instructions for organising this matter. They don't make it.
When the big bang cooled to the point where matter could form, the whole universe was pretty much 100% hydrogen (and empty space). Every atom in the universe which isn't hydrogen was generated in the hearts of stars formed by clumps of hydrogen which coalesced into nuclear fusion reactors and generated new heavier atoms.
Its from the fact that all matter in the universe is constantly being reused. We arnt just what we once were. We're also what we eat, drink, breath, and simply absorb through our skin. And those things are made up of molecules that come from else where and so on. Considering that the majority of all the matter in the universe is in stars, it makes perfect sense