The Story of DNA

A DNA strand contains a complete representation of everything about our physiology. It also contains instructions on how to form our body by repeated divisions of a single cell. Each cell needs to know when it should, split into two, split into different kinds of cell for tissue differentiation. Cells also need to know when to stop growing because the body or organ is mature, and when it needs to replace tissue lost by injury. All of that is encoded into one molecule. So in other words the benefit of squeezing a lot of data into our cells is that our cell knows what to do and when to do it.
DNA contains instructions on how to make proteins. Each cell uses its own DNA to make proteins which it needs to do its job. Other parts of the DNA tell the body what cells to make, but even this job is done by other cells, which split, differentiate and turn into organs because of the instruction in DNA. That's why it is in every cell, and not in some central place, because every cell was split off from some other cell that also had a full set of DNA.
The 4 bases of DNA lead to very complicated proteins according to set rules. DNA has four bases, represented by the letters A, T, C and G; the initials are of the chemical names for those bases. They are arranged in groups of three, each group being called a codon. So ATA is one codon, CGT is another. Even with only four letters, you can get many codons and thus very complicated instructions.
I'm no expert, but our body grows by producing cells. They DNA code is transmitted to each cell so they all will match. If not, we would have brown hair on one side of your head and red on another or one arm would be longer than another. That is simplified