Human Design describes a person using a diagram, the bodygraph: nine centers, 36 channels and 64 gates, calculated from the date, time and place of birth. The basics explain how this chart is created, why there are two calculations (Personality at the moment of birth, Design about 88 days earlier) and what the colors black and red mean.
They also cover the terms that appear in every reading: definition (single, split, triple, quadruple), defined and open centers, hanging gates, strategy and authority. Once you understand these, you can read any chart, including your own.
It helps to read with your own chart next to you: calculate your chart for free. The audiobook begins with exactly these basics, and part 1 is completely free.