Mayo Clinic has made a short and informative video about genomic sequencing. It starts with the basics, noting that all the cells in our body carry DNA in their nuclei, and that DNA has to be extracted, usually from a blood sample, in order to reach the genomic material inside. It has a helical structure that looks like a ladder when it's stretched out, and is made up of nucleotide bases.
Those nucleotides, adenine, thymine, guanine, and cytosine, comprise the sequence of DNA, and they pair in a special way. The DNA can be read after each base is tagged, and ultimately, you end up with the genomic sequence. While that can tell us a lot about some conditions, there are still many mysteries in our genome.