What are we referring to when we say ‘simple organisms’

What are we referring to when we say ‘simple organisms’ or ‘complex organisms’?

These terms are used to classify organisms according to their evolutionary history. Simple organisms refer to those organisms that have simple structural and functional organisation and are considered primitive, whereas complex organisms refer to those organisms that have higher and complex levels of structural and functional organisation. These are more advanced and said to have arisen from simple organisms.