Mutualism in biology is like when two different living things help each other out and both benefit from it. It's like sharing toys with your friend and both of you are happy playing together.
In nature, there are many different kinds of living things that have mutualistic relationships. For example, bees visit flowers to collect nectar and pollen, but in the process they also help the flowers reproduce by transferring pollen from one flower to another. This helps both the bee and the flower because the bee gets food and the flower gets pollinated.
Another example is the relationship between clownfish and sea anemones. Clownfish live in the anemone's tentacles and get protection from predators, while the anemone gets food from the leftovers of the fish.
Mutualism is important because it helps different species survive and thrive in their environment. It's like having a best friend who helps you out when you need it, and you help them too.