Sometimes when I go to a bar or a concert, someone in the audience starts yelling “FREEBIRD” I think it’s an insult, is it ? what are they trying to say ?
Maybe they are requesting that the band play “Freebird,” the song.
Or maybe it’s some sort of joke on the type of person who would request Freebird that neither you nor I are in on.
Right Said Fred.
Just to update, this site here supports my second hypothesis.
Sometimes they want to hear the song, sometimes it’s a joke.
I’m married to a musician, and he’s been in the business for almost thirty years now. He’s a guitar player, and he’s awesome. We’ve been together for about eight years now, and I’ve heard him play “Freebird” precisely once. Tore that puppy up, he did. But…the first time I ever asked him to play it, I did it in the joking fashion that you see in the bars. Held up my lighter, and hollered “FREEEEEEEBIIIIIIIIIRRRRRRRRDDDDDD!!!”
It was the one and only time in our relationship he’s ever looked me square in the eye and said “f*** you.” 
If you’ve got some time, listen to this entertaining piece from NPR back in August: Freebird
Freebird is a tired old tune that most nightclub musicians have had to perform about eight hundred billion times, give or take. Requesting Freebird used to be an inside joke among musicians. (“What do you want to play next?” “How about Freebird?” “Oh, Jesus, not that thing again!”)
People in the audience who knew musicians used to shout it out to let them know that they were in on the joke too.
Now the joke itself is tired and old and not funny anymore. Most musicians will just give you the finger if you shout it out now.
It’s a joke because Freebird was a concert staple of Lynyrd Skynyrd and was an overly long and drawn-out piece. Before Freebird, people used to yell “Whipping Post!” I heard it at a Dire Straits concert in the mid 80s.