Camera bodies and lenses are factory calibrated by default, but sometimes they require further adjustments to achieve better results when focusing. With the Nikon's AF fine tune feature on supported models, you can manually make precise adjustments to fix any focusing problems.
In this 12 minute video, photographer Steve Perry explains the auto-focus calibration techniques in depth, particularly on Nikon bodies such as Nikon D5, Nikon D500, and Nikon D7500, which have the latest Nikon AF system. This technique may also apply for the new Nikon D850, as it will most likely have the same AF system. Unfortunately, this technique is not applicable to older Nikon camera bodies, including Nikon D810.
AF fine tune is not a mandatory process for everyone, however, if you are experiencing front or back focusing issues, then you should calibrate your AF. By the way, need for calibration doesn't mean that you have a faulty camera body or a lens, it’s just a tweak that you should apply once for specific camera – lens combinations. If you want to learn more about back or front focusing, don't forget to read David Strauss' post about focusing issues.
You can either use pro-calibration tools, such as LensAlign, to fine-tune your system, but you can also achieve successful results with this step-by-step guide, that includes all camera settings and AF target use. You can also download the target image from this link, to give it a try with your own setup.