Active transport moves substances from a more dilute solution to a more concentrated solution. This is against the concentration gradient, the opposite direction to diffusion. Because it goes uphill, it needs energy, and that energy comes from respiration.
Plant root hair cells use active transport to take up mineral ions from the soil. The soil water is very dilute, so the ion concentration is lower outside the root than inside the cell, and diffusion alone could not move the ions in. Plants need these ions for healthy growth.
In the gut, active transport absorbs sugar molecules from the lower concentration in the gut into the blood, which has a higher sugar concentration. The sugar is then used for cell respiration. This links to cell specialisation, because cells that do a lot of active transport have many mitochondria to release the energy they need.
The three processes can be compared. Diffusion moves particles down a concentration gradient and needs no energy from respiration. Osmosis is the diffusion of water only, through a partially permeable membrane. Active transport moves substances against a concentration gradient and does need energy from respiration.
