It's sometimes a greeting, or sometimes a description of a person who's friendly, likeable and genial.
definition:
http://www.merriam-webster.com/dictionary/hail-fellow-well-met
Some quotes from literature:
And at first he sings small, and is hail-fellow-well-met with Sheamus -- that's James of the Glens, my chieftain's agent.
Kidnapped by Stevenson, Robert Louis View in context
Down there they talk of the heavenly King - and that is right - but then they go right on speaking as if this was a republic and everybody was on a dead level with everybody else, and privileged to fling his arms around anybody he comes across, and be hail-fellow-well-met with all the elect, from the highest down.
Captain Stormfield's Visit to Heaven by Twain, Mark
origin:
http://archives.stupidquestion.net/sq71904.html