So diffusion is a form of passive transport, right, so the particles are moving with the concentration gradient from a region higher density to a region of lower density. This happens naturally as an effect of molecules colliding, so they do not require the cell to make an effort and use up energy. Active transport requires the cell to use energy, because in active transport the particles are moving from a region of lower density to one of higher density, against the concentration gradient, this is not natural so the cell needs to make an effort and use energy if it wants the particles to pass through its membrane.
