A keeper league is one that enables owners to retain certain players from season to season. How many players can be kept is at the discretion of each league, and oftentimes retaining a player comes at a cost -- such as forfeiting a draft pick.
Using keepers is common in dynasty leagues that play out over several seasons. Usually, players can only be kept for a certain amount of time before they must be released back to the player pool available to be drafted prior to a given season.