A bud is a cluster of single female flowers.
As you can see in the pic, this cola is composed of several sub-units that are will fill out before reaching maturity. In this phase, the plant has finished the stretching phase and is developing bud sites.
Depending on the strain, bud development may start in the middle of its flowering period.
Every single sub-unit is itself a cluster of single female flowers. New Flowers are formed throughout the flowering period. You can see new flower production on top of each sub unit.
In this pic, you can see (above the yellow lines) where the single sub-units develop new single flowers. You can recognize them because the new pistils in formation have tiny stigmas.
Let's see a single flower!
This pic is a pre-flower but is alright for our purposes. It explains how a single female flower formation occurs.
This single flower is called a pistil and the two hairs coming out of it are called stigmas. Male pollen reaches the stigmas and make their way to the female egg cell which is located inside the pistil. This pollination process will produce the seed. Growers tend to prefer seed-less (non-pollinated) sensimilla!
Resin is produced on female flowers and on the leaves near them. It is produced by a particular structure called a glandular trichome.
In this pic you can see on the leaf that resin is produced in some kind of ball over a thin neck. if you have a magnifying glass, you can look in the ball and see color changes (from transparent to white to gold/brown) and determine when resin production and THC % has peaked. Rough handling of buds will break the trichomes off.
Let's see now how a pistil evolves during the flowering period. Above the yellow line you can see a young pistil. It is thin and has white hairs.
Here is a pic of an older pistil. The body has enlarged. The stigmas are still looking for some pollen. The small leaf under the pistil is called a stipule (Stipules are more evident with pre-flowers.)
This is probably two or three days later, as you can see hairs are becoming (starting from the top) brown/red. Probably at this stage it can still receive male pollen.
In this pic I've underlined three pistils that are as mature as the one in the previous pic (more or less). Other stigmas have dried up, probably due to rain and wind. In these conditions some stigmas lose their vitality and start to dry and to become brown/red. This is not a sign of ripening, just a normal event
On the left of this pic, you can see two pistils fully mature. The stigmas are totally colored and dried. The body of the pistil is now fat and is beginning to lose its turgidity. On the right you can see a cluster with pistils in various stages.
The pistil after a couple of days. It has lost its turgidity and probably is not receptive to pollen. it is dying. When all 70/80 % of all the pistils of a bud are at this stage, resin production has slowed if not stopped.
The maturation of the pistils in a bud is not synchronized but gradual. As you can see in this pic, you have lots of pistils at different stages of development. See the dimension of the bodies. At harvest, you will probably see lots of brown hairs, but also some white at the top of each single sub-unit where the new formed pistils are.
When stigmas are dried and pistil is dying , the hairs will easily break and fall down.