This may get way off topic, and may create a huge answer. I'm quite sorry. Movement is passive when the molecule does not need to use energy in order for it to enter the cell membrane, an example of this would be the simple and facilitate diffusion, and osmosis. Diffusion is the movement of molecules from a high concentration to lower concentration, like in the diffusion of oxygen into red blood cells from the alveoli as oxygen concentration is greater in the alveoli than in the red blood cells.
Movement is active when the molecule needs to consume energy or ATP in order for it to enter a cell membrane, an example of this would be the sodium-potassium pump found in neurons.
