DICTIONARY.COM has flocking to mean the following
1:A group of animals that live, travel, or feed together.
2:A group of people under the leadership of one person, especially the members of a church.
3:A large crowd or number: had a flock of questions.
The group of animals could be Birds or fish that move and stay together as a group by mimicking each others movements
For Artificial Intelligence AI it means "behaviour that mimics lifeforms. The example I gave of flocking above about the birds is the main type of behavior that this represents"
Flocking in AI is in the form of mathematical algorithms(expressions) that represent rules for how objects(flockmates) behave.
The 4 simple rules
This keeps the flockmates from crowding the other flockmates so that they dont bump into each other.
This makes each flockmate want to move at the same speed and in the same average direction as each other.
This is rule keeps are the flockmates the tendency to keep together so that if they stray a certain distance, they will want to move towards the average position of all the other flockmates.
4. Obstacle Avoidance
This keeps the flockmates from colliding with other objects that are not other flockmates. e.g a mountain
For futher study into artificial intelligence flocking. Check out Boids by Craig Reynolds available at URL http://www.red3d.com/cwr/boids/