What is bee brood?
Bee brood means the entire bee developmental stage from egg until the time when the growing bee emerges from its honeycomb cell as an adult bee. Open brood refers to the earliest part of this stage, during which an egg is laid and covered with food (royal jelly). Closed brood refers to the later part of this stage when those cells are covered over (capped).
In this photo, the cells to the left are older capped brood. Moving rightward, there is open brood, which will soon get capped. To the right of that is brood that is quite young, up to only a couple of days old.