Mutualism is a type of symbiosis. A mutualistic relationship is when two organisms of different species "work together," each benefiting from the relationship.
Legumes have a symbiotic relationship with bacteria called rhizobiam.The bacteria live in the plants cells, located on the root, and form nodules. These nodules convert nitrogen gas in the soil to usable energy for the plant. In return the bacteria have a safe place to live and reproduce.
Rhizobium helps fix nitrogen that is used up by legumes in the soil as legumes harbor rhizobium in their root nodules.