What does Carl Rogers mean by "organism?" I know he is referring to people, but I don't understand why. How does the person centered approach...

Rogers believed that all living things are organisms.. including humans and that just as plants need certains conditions in order to promote their growth, so do humans.

The conditions that Rogers thought the human organisms need in order to promote their growth are the core condition of his 'person centered' model. (Rogers is referring to emotional rather than physical growth.)The conditions are

Empathy

Congrence (genuineness - from others and ourselves)

Unconditional positive regard (to feel that we are loved or approved of unconditionally by someone)

Hope this helps.