Mutagen is a special brand of chemical created by Techno-Cosmic Research Industries. When a living being is exposed to it, his/her strength and size increases to an amazing degree, or they gain assorted superpowers.
Luke Cage was part of an experiment that involved injecting Mutagen into him. The Mutagen gave him superhuman strength and an unbreakable skin.
As a little kid, Matt Murdock was accidentally splashed with Mutagen. This permanently blinded him, but gave him superhuman hearing, superhuman smelling and immunity to poisons.
Jessica Jones was also splashed by Mutagen, which gave her superhuman strength, superhuman speed, invulnerability, and the ability to resist mind control.