Brain is a new system that has been implemented in the Village & Pillage update to expand the AI for certain mobs such as the Villager. nowadays, the entities using Brain are: Villager, Piglin, Axolotl, Goat, Tadpole, Frog, Warden and Camel.
it's an interesting system because unlike the goal system, it works using activities, sensors and memories! While goals run using tasks and priorities, Brain runs different behaviors for each activity.
Each Activity has defined behaviors that will run while the activity is active, these can be defined with memories so the entity knows the conditions to run certain tasks.