the tongue of the neck is the part that sits in the neck pocket... it's kind of a Stratocaster or a bolt on term... Sometimes moisture gets into the 4 or 3 screw holes on a bolt on neck style guitar and causes that part of the neck to expand... raising or warping that part of the neck... giving it the name "raised tongue". It can be pretty serious since if that part of the neck warps... there's no real way to fix it except by taking off the frets in that area... shaving the fretboard and evening it out... then refretting. Sometimes the swelling may occur and suddenly return to its original state... But in most cases a raised tongue is permanent and can't be counteracted by a truss rod adjustment... the way that most linear warping (warping where the neck bows toward or away from the bridge) closer to the headstock can be counteracted... Raised tongues are normally more prominent in one piece maple necks... I don't know why... maybe something to do with the rosewood acting as a brace for the maple and it helps the maple keep its shape at the tongue area..... Some luthiers call this a raised heel as well. I picked the term up from one of Dan Erlewines books though...