It comes from Greek and it means to make smaller. The use is similar to its Greek origins. It denotes any form of description that is designed to lessen the value or importance of a thing. It can seem similar to litotes but while litotes is more sarcastic, meiosis is far more direct. An example of meiosis might be “It is only a scratch“. In the case mentioned, the wound the speaker suffered is probably a serious or grave one, but he isn’t discouraged by it or doesn’t want to let his opponents know just how much damage he took. That is why he or she used meiosis to both encourage himself and discourage his or her opponents.