Cannabis plants have hormonal systems that regulate their growth in response to external stresses. The concept of crop steering is essentially the exploitation of these hormonal responses, by manipulating a controlled environment to achieve a desired outcome - like bigger yields.
When a plant puts most of its energy into producing leaves, and not much into flower formation, the plant is considered to be in vegetative growth. When a plant is focused on the development of flowers, it is considered to be in generative growth.