In this interaction, one creature is the predator and one is the prey. Without external influences such conditions are always in balance: hunting reduces the prey population, consequently there is less food for the hunter and also his population decreases. Now the prey is undisturbed and its number increases again, so the cycle goes on and on. Such interactions are mainly known from the animal kingdom, but such processes are also known in microorganisms. For example, protozoa eat bacteria or bacteria are infected by viruses.
Explanation for children: A predator-prey relationship describes the relationship between a hunter and his prey. For example, the fox is the predator and the rabbit the prey. The foxes hunt and eat the rabbits, so there are fewer rabbits. Because there is now no food for the foxes, there will soon be fewer foxes. Fewer foxes means less stress for the hares, the hares multiply again. Now also the foxes multiply again, because there is more to eat again. One can imagine this relationship as a cycle that is always in balance. But if you were to intervene from the outside by hunting foxes, for example, you would lose your balance. We know this relationship from the animal kingdom, but there are also predators among microorganisms who attack other microorganisms.