CRISPR-Cas9 allows geneticists and medical researchers to edit parts of the genome by removing, adding or altering sections of a DNA sequence. Cas9 is an enzyme that cuts the DNA strand where the scientist wants it cut. There is a also guide RNA that guides Cas9 to the part of the DNA being cut. The DNA realizes that is damaged and tries to repair it but this is where the scientist comes in and adds in or removes the gene in question. CRISPR/Cas9 was discovered in bacteria that use it to fend off viruses.
This technology is easier than some other biotechnology and more accurate. For example, one way to change a genome is by radiation causing mutations. Radiation is not controlled so many genes could be mutated and it could be genes you didn’t want mutated. With CRISPR/Cas9, you can pinpoint exactly which gene you want mutated and CRISPR/Cas9 will directly mutate that gene without disturbing the rest of the genome. Another method is gene targeting but that is much more expensive than CRISPR/Cas9. As of right now, it is considered the fastest, cheapest, and most reliable way to gene edit.