Trichomes are small appendages that look like hairs, they are produced by marijuana and other plants. Some trichomes on marijuana plants develop a resin gland at the top. These trichomes are a rich source of THC, and monitoring them will allow you to best judge when the plants are ready for harvesting.
On marijuana plants, trichomes with resin glands form in the highest concentration on buds. They start out clear, turn a milky color, then turn amber (light brown). To best judge when to harvest, keep an eye on the trichomes on the buds on each plant.
Do you see the size of those trichomes on that Marijuana plant?!