Trichomes are the tiny, hair-like glands that cover cannabis flowers and the small leaves around them. Up close they look like little mushrooms with round heads, and from a distance they give good flower its frosty, sugar-coated look. They are also what makes cannabis sticky.
Those little heads are where the plant makes and stores most of its cannabinoids, like THCA, and its terpenes, which create smell and flavor. In other words, trichomes are where most of the action is. Growers watch them closely near harvest: the heads change from clear to cloudy to amber as the plant matures, and many growers time the harvest by that color.
Trichomes are also the starting point for many concentrates. Kief is simply loose trichomes that fall off dried flower. Hash is trichomes collected and pressed together. Ice-water hash separates trichomes with cold water and agitation, and rosin is often pressed from that hash. Solvent-based extracts dissolve the trichome contents instead.
Because trichomes are fragile, rough handling, heat and time can damage them. That is one reason careful curing, airtight storage and keeping flower away from heat and light help it hold onto aroma and quality.