DNA is an abbreviation for deoxyribonucleic acid. In eukaryotes, DNA is present in the cell nucleus and in prokaryotes it floats around in the cell lumen. It is found in each of our cells and contains everything that makes us who we are. You can think of it as a little book that says "blue eyes", "brown hair", "long fingers", and so on. When we grow and our cells divide, DNA must also be doubled. To make sure that no mistakes are made in such an important task, we have many mechanisms that take care of our DNA and are able to repair it.
Explanation for children:
An important part of cells is DNA. In eukaryotes it is located in the cell nucleus and in prokaryotes in the cell fluid, because they do not have a cell nucleus (orange). You can imagine the DNA as a construction manual that tells us, for example, what eye color we should have or how a muscle is built. When we grow, our cells grow by dividing from one cell to two cells. The DNA needs to be doubled so that there is always a copy of the DNA in each cell. Nothing must go wrong in this process, which is why there are many mechanisms that take care of the DNA.