CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) is a genetic engineering technique that allows for precise editing of the genome. One application of CRISPR is gene knockout, which involves disabling or "knocking out" a specific gene in an organism.
The process of gene knockout with CRISPR involves three main steps: designing a guide RNA (gRNA) that targets a specific location in the genome, delivering the gRNA and a Cas9 enzyme (which acts as a molecular scissors) to the target cell, and then allowing the cell to repair the cut in the DNA. When the cell repairs the cut, it can either join the cut ends back together, resulting in a non-functional gene, or introduce a mutation that disrupts the gene's function.
This technique can be used in a variety of organisms, including bacteria, yeast, plants, and animals, and it allows scientists to study the function of specific genes by observing the effects of their absence. CRISPR-based gene knockout is a powerful tool for understanding the genetic basis of disease and for developing new therapies. 
It is important to note that CRISPR-based gene knockout, like any genetic engineering technique, has the potential to produce unintended or harmful effects on the organism, so it should be used with caution. The coupled Cas9 will cause a double stranded break in the DNA. Following the same principle as zinc-fingers and TALENs, the attempts to repair these double stranded breaks often result in frameshift mutations that result in an nonfunctional gene. Non invasive CRISPR-Cas9 technology has successfully knocked out a gene associated in depression and anxiety in mice, being the first successful delivery passing through the blood–brain barrier to enable gene modification.