So, what is biohacking? A hacker is someone who wants to gain complete control of the system they are dealing with be it technical or social. A biohacker, therefore is someone who wants to gain complete control of their own biology.
OK then, what is ketosis? Ketosis is a metabolic state in which the liver produces small organic molecules called ketone bodies in significant levels for the body to be able to use them as a primary aerobic energy source. If you are interested in the full details of how this all happens, the best (and most understandable) explanation can be found on a blog 
by Dr Peter Attia, who has been biohacking himself on this for some years now and undertakes very detailed measurements.