LNIB= Like New In Box (usually for items that were opened or maybe just tested. As close to new as you can get without it being new and sealed.)
NIB= New In Box
sig and signature is usually people who have their threads to sales or helpful links in their signatures, usually separated by a bar from their post. Honestly in the time it takes to say "sig" they could just copy their own link, but people do it. So if someone says sig, just check the writing under their post.
Bump is just to avoid threads getting put back at the top with no real update or extra content. It's always just better to say something new in the post than just a "bump".