Hey there!
DNA is short for deoxyribonucleic acid. It's in every cell in every single part of your body. Although small, it contains the instructions to everything you do and everything you will ever do. They are made of nucleotides, made of sugar, phosphate, and nitrogen
For example, strands of DNA can determine your genetic makeup, your facial features, your body features, diseases you can get, diseases you're immune to, how you'll age, how you'll grow, your ancestral tendencies in humans, and many other things. It determines many, many things about you.
DNA is not only in humans. It is in bacteria, viruses, and other organisms. Like in humans, it determines many aspects of how they're made up.
Hope this helps!