The BRP (Biomechanical Reference Point) is the point, common to all saddles currently on the market, where the saddle reaches a width of 70mm. Defining the BRP allows you to have a common denominator from which you can calculate all the setup measurements for your saddle. (saddle height, setback, saddle-handlebar distance and saddle-bar stem drop).
The reference HB, corresponds to the point of intersection between the bike's handlebars and stem. Defining the HB point makes it easier to correctly transfer the measurements suggested by your bike-fitting to the bike.