Michael,
When a thread goes unanswered for a long period of time someone may add a post just to 'bump' it back to the top of the list of Latest Posts. In that case, they typically just post "Bump" since they are not adding anything substantive to the discussion.
And you are correct, 'LF' is short-hand for Looking For. You may also see 'WTB', meaning Want To Buy; or 'FSOT', meaning For Sale Or Trade.