What a gate is
Gates are the 64 points on the bodygraph; each corresponds to one hexagram of the I Ching and one sector of the zodiac wheel. A gate is the smallest building block of a chart: a specific energy, quality or tendency. Gates sit in the centers, and every gate has its own six lines.
How they are activated
A gate is activated when a planet stood in it at the moment of birth (or about 88 days before birth, for the Design). Black activations are the conscious Personality, red ones the unconscious Design. A single gate without its partner is called a "hanging" gate: it looks for completion in other people or in the transits.