I'm not an expert. But basically what 23andme tells you are what parts of your DNA deviate from the reference genome. If you have certain mutations in certain genes, then those genes (which correspond to enzymes) have a defect: the corresponding enzymes may not work as expected. For example, if 23andme tells you that you have a defect in a gene called BCMO1 (which encodes an enzyme that converts carotenes into retinol), then Nutrahacker could tell you that you might benefit from taking a retinol supplement. (The general case is more complicated than described here, as most genes may not be hacked around by adding nutrition directly, so a more careful approach is needed).

There is a lot of focus on the link between mental disorders and nutrition. For example, people with depression, anxiety, and autism have been helped by addressing their nutritional needs. In some cases, these disorders may be "late-onset disorders", they appear later in life. So even if you have no disorder right now, it might still be a good idea to optimize your diet.