It refers to a piece of gear that is the best piece you can equip on a specific place (like a weapon, or a piece of armor).
p2. Yeah, I saw that, they are BiS for resto droods.
2) Short for "bisexuals."
Jeff: "Reminds me of your mom and sister last night."
Jeff: "I hit those bis from the back REAL hard, bitch!"
Rocky: "Awwww sniggity snizzap."