Final answer:
Passive transport is the movement of substances from an area of higher concentration to an area of lower concentration until equilibrium is achieved, requiring no cellular energy.
Step-by-step explanation:
Passive transport involves the movement of material from an area of high concentration to an area of low concentration. This natural phenomenon occurs without any input of energy from the cell. An example of this process is diffusion, where substances move along the concentration gradient until an equilibrium is reached, meaning the concentration is equal across a space.