Reflexive pronouns are used when a subject noun takes itself as its own object. Some common reflexive pronouns are myself, ourselves, oneself, and herself.
Certain tomopterid worms defend themselves by squirting out a bioluminescent fluid.
--Bruce Robison, "Light in the Ocean's Midwaters," Scientific American (modified)
Reflexive pronouns can also be used to emphasize the subject. In that case, they are called intensive pronouns.
I did it myself.
Use intensive pronouns sparingly, since they can make your document wordy.