Trichomes are the portion of the cannabis plant that all cannabinoids and terpenes are derived from. They are the tiny crystals you see when you examine mature wet or dried flower, and are the primary source for the essential oils found in cannabis. As the marijuana plant matures in flower, trichomes are formed on the outer surface of the plant, including the buds and leaves of the plants.
As the plant gets closer to harvest, these trichomes often go from clear, to a more amber color, signaling the plant is ready for harvest. It is believed that the more trichomes that are on the marijuana plant, the more potent the finished product will be.
The pictures below show trichomes in their infancy stage vs maturity. Notice how they have matured into a golden, amber color right before harvest.