When we say ‘simpler organisms’ or ‘complex organisms’ what are we referring to?

When we say simple or complex organisms we are talking in terms of evolutionary history of an organism. A simple organism is considered to be primitive and has simple thallus organisation. The level of complexity of metabolism is also low. On the other hand a complex organism refers to a more evolved form forming higher levels of structural and functional complexities. They are believed to have arisen from simple organisms.