People who ride bicycles without chain guards while wearing loose pants sometimes roll up one leg to keep fabric out of the chain. As a fashion style, this emerged as a trend in the 1990s. It may have been popularized by LL Cool J or have had some gang significance.
This fashion statement, which depends on loose pants, has been used in many ways. Freemasons use a rolled trouser leg in some parts of their rituals. Jamaican cyclists sometimes roll up one leg to avoid a greasy chain. The style may also have, at one point, signalled a bicycle messenger who had drugs for sale.