Miso is a traditional Japanese seasoning. It is produced by fermenting soybeans with salt and kōji and sometimes rice, barley, seaweed, or other ingredients.
At its most basic, miso is a fermented paste that's made by inoculating a mixture of soybeans with a mold called koji (for you science folks, that's the common name for Aspergillus oryzae) that's been cultivated from rice, barley, or soybeans.
Miso is the ultimate reference point for the flavor sensation known as umami. The paste and the soup have a deep savory flavor, with toasty, funky, salty-sweet richness.
This umami flavor forms the base of a lot of everyday Japanese cooking.