Food animals use energy for movement and for keeping warm, and this energy is transferred to the environment. The efficiency of food production can be improved by restricting this energy transfer. When less energy is lost, more of the animal's food goes into growth, so more food is produced for the same amount of feed.
This can be done in two ways. Limiting the animals' movement, for example by keeping them in small pens, means they use less energy moving about. Controlling the temperature of their surroundings, for example by keeping them in warm buildings, means less energy is lost as heat to the environment.
Some animals are also fed high protein foods to increase their growth.
These intensive methods have advantages, such as cheaper food and more of it, but some people have ethical objections to them. They argue that restricting movement is bad for the welfare of the animals. You may be asked to evaluate the advantages and disadvantages of modern farming techniques.