A cell membrane separates a cell's interior from its surroundings while still needing to let specific substances cross in both directions, and it manages that using three genuinely different mechanisms. Diffusion lets a dissolved substance flow from where it's more concentrated toward where it's less concentrated, and osmosis is the same basic process applied specifically to water crossing a selectively permeable membrane. Both happen for free, driven purely by the concentration gradient itself. Active transport is different: it spends the cell's own stored energy to push a substance against its concentration gradient, moving it from low to high concentration, uphill, whenever the cell genuinely needs that.
Diffusion and osmosis run downhill and need no energy investment from the cell
Diffusion happens because molecules are in constant random motion, and that random motion, averaged across huge numbers of molecules, naturally spreads a substance from a region of higher concentration to one of lower concentration until the two sides equalise, with no directional push required from the cell itself. Osmosis works by exactly the same underlying logic, just tracking water specifically as it moves across a membrane toward the side with a higher concentration of dissolved solutes, diluting that side and equalising the two solutions' overall concentration. Both processes are entirely passive: the cell doesn't spend any of its own energy driving them, since the concentration gradient itself supplies all the "push."
Active transport spends real cellular energy to move substances the "wrong" way
Many substances a cell actually needs sit at a lower concentration outside the cell than the concentration the cell wants to maintain inside, meaning simple diffusion would move that substance the wrong way entirely, or not move it usefully at all once concentrations equalised. Active transport solves that problem directly, using specialised membrane proteins that consume the cell's own chemical energy specifically to push a substance against its natural concentration gradient, moving it from where it's scarce to where it's needed even though diffusion alone would never do that on its own.
What we're still unsure about
That diffusion and osmosis move substances passively down a concentration gradient, while active transport spends real cellular energy to move substances against one, is extremely well established, thoroughly confirmed cell biology. What's more genuinely an active area of ongoing research is exactly how the many different specialised transport proteins embedded in a real cell membrane are regulated and coordinated together, since a cell manages dozens of different substances crossing its membrane simultaneously through different specific channels and pumps, and researchers continue to work out the precise regulatory signals that determine which transport proteins are active at any given moment, rather than that full regulatory picture being completely mapped out.
This sits inside Cell Membrane & Transport (Diffusion, Osmosis, Active Transport), one of seven topics in Cell Biology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.