Cell Membranes as an active barrier - (Sep/23/2018 )

Dears,

What do you understand by the phrase 'cell membrane is an active barrier'?  Does it mean it uses energy in the form of ATP to transport molecules across or it is selective permeable because it controls which membrane transporters are incorporated in the bilayer?

This phrase is part of an undergraduate essay title "Cell membranes are active barriers. Discuss'

Cell membranes contain hydrophobic portion inside bilayer and the hydrophilic portion outside. Molecules greater in size do not cross membrane but o2, co2 pass through the simple diffusion. For large size, hydrophilic molecules membrane channels are needed. and if the movement is against the concentration gradient then ATP is involved.