DNA is a long chemical molecule stored in almost every cell of the body. Its job is to hold instructions. You can think of it as a very long book written in a four-letter alphabet, where the letters are the chemical building blocks adenine, thymine, guanine, and cytosine — usually written as A, T, G, and C.
The order of those letters is what carries meaning. A stretch of DNA that spells out the instructions for one product is called a gene. The rest of the DNA is not empty filler; much of it helps control when, where, and how strongly genes are used. Because the same DNA is present in nearly every cell, what makes a liver cell different from a nerve cell is not which DNA it has, but which parts of the book that cell chooses to read.
This is why DNA matters for medicine. If a medicine is going to change how the body behaves, it will usually do so by acting on a product that DNA ultimately specifies. Reading DNA is therefore a way of reading the list of things a medicine could act on.