What does LB mean?

[Mhaeric]

I am as new as it comes to MMORPG's and while doing a duty someone asked me to LB

I don't know what that means.

Could someone clarify?

Here's a very detailed explanation about how limit breaks work: http://www.ffxivinfo.com/content/limit-breaks.php

One note is that Arcanists, Summoners and Scholars are missing from that guide. Arcanist and Summoner limit break is the same as a Black Mage. Scholar is the same as White Mage.