Substances are transported passively down concentration gradients. Often, substances have to be moved from a low to a high concentration - against a concentration gradient.
Active transport is a process that is required to move molecules against a concentration gradient. The process requires energy from respiration.
Plants need to absorb mineral ions from the soil to keep healthy. They need to absorb them from a low concentration in the soil to a higher concentration in the plant. This is against a concentration gradient, so it will not happen naturally like diffusion and osmosis. Plants therefore use active transport to absorb mineral ions into root hair cells.
Animals, including humans, need to absorb all glucose molecules from their food. This is taken from the gut wall, inside the small intestine, into the blood. There is more glucose (a higher concentration of glucose) in the intestine than the blood after having just eaten a sugary meal. So, the glucose moves from high concentration in the small intestine to lower concentration in the blood by diffusion.
However, after a short while there is the same or more glucose in the blood, so there is a low concentration of glucose in the intestine. Now glucose cannot move into the blood by diffusion. This is against a concentration gradient, so it will not happen naturally. Animals therefore use active transport to absorb glucose into the blood under these conditions. The process requires energy produced by respiration.