That's probably from an mcpe server host. It most likely only applies to MCPC servers, so you can ignore it.
Wrong. Online mode is there because the server.properties file should emitate the one of PC servers, on them, online mode means if the server should connect to the Mojang API to see if the players are using valid MC accounts. At the moment it is not in use in PMMP, however, in the future it may be used with Xbox live auth [citation needed]