Mutualism is the way two organisms biologically interact where each individual derives a fitness benefit.
It is a subdivision of intraspecific interaction. One specie needs the other but can live without it. Both of them benefit from the other. For example, a buffalo lets the oxpecker put on him to eat the flies and release from them, but the bird also benefit because can eat something that seem to it to be good. Let’s see the video of a enjoyed way:
I have taken the information from the lesson and the first part (the definition) from wikipedia.