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

(N/A) $\rightarrow$ When we say simple or complex organisms,we are referring to the evolutionary history of an organism.
$A$ 'simple organism' is considered to be primitive and possesses a simple thallus organization,with a low level of metabolic complexity.
On the other hand,a 'complex organism' refers to a more evolved form that exhibits higher levels of structural and functional complexity. These organisms are believed to have evolved from simpler ancestral forms.