Astrologers rarely draw the Earth on a chart, because from the Earth's own point of view it has no position in the sky. Human Design puts it in as a matter of principle. A life force with nothing to ground it is not livable, and the Earth activation shows where your stability comes from.
On the Personality Side
Your Personality Earth is your conscious balance. It is how you steady yourself when the theme of your Personality Sun is running strongly, and the quality you lean on to bring that theme into ordinary life. When you have integrated your Personality properly, this is the gate that shows the world you are grounded rather than merely bright.
On the Design Side
Your Design Earth is the body's own stabiliser. It works without your knowledge, in the way you physically hold yourself together when life presses on you, and it shows the terms on which your form has come to grips with the material world. People who know you well often see this gate in you before they see the Design Sun, because it is what steadies you under pressure.
Reading It In Your Chart
Find the Earth glyph on each side of your chart and note the gate. Then look across to the Sun on the same side: the two gates will be opposites in the wheel, and often they read as a theme and its counterweight. Along with the two Suns, these four gates make up your Incarnation Cross, so the Earth is not a minor placement. It is one of the four pillars of your life theme, and the one that keeps the other three standing.
The Mother Principle
Human Design gives the Sun and the Earth the roles of Father and Mother not as a comment on parents but as a description of two forces. One radiates, one receives and gives form. Every gate in your chart is coloured by one of these two, and the Earth gates are where you find out what kind of ground you are made of.