means The selling of goods directly to individual customers in small quantities, as opposed to wholesale.

from From Old French 'retaillier,' meaning to cut off or clip a piece, built from 're-' plus 'tailler,' to cut (the same root that gives us 'tailor' and 'detail'). The idea was selling in small pieces snipped from a larger whole — buying a bolt of cloth wholesale, then 'retailing' it yard by yard. The word's been doing this work in English since the 14th century, and that cutting sense survives oddly in the phrase 'to retail a story' — to pass it along in pieces.