Here's a summary of the numerous answers I got at a hockey board:

The turnbuckle is where the glass starts (or ends--whichever) at the ends of the benches.

There are usually open supports (called stanchions) at the end of each [players'] bench, and also sometimes around penalty boxes. At the upper levels [of pro hockey] the penalty boxes are closed in but at most youth rinks there is no glass covering the boxes so it creates another area where the stanchion is exposed.

These are very dangerous areas to take a hit because you are hitting a SOLID stationary thing and most of the time you can't get your hands up to protect yourself, resulting in your head or upper body taking the entire blow.

At most arenas now they have put up some sort of padding to cover up the exposed metal. Does this padding really help? In most cases not enough. I guess it is better then hitting the metal but it doesn't soften the blow enough.

Yeah and where the glass starts (or ends) there is this foam or some kind of padding along the edge of the glass so if a player goes into that, there is somewhat of a cushion and not hitting the edge of the glass...

You know, like in wrestling where they bounce their opponents head off the turnbuckles, the thing in the corners where the ropes connect... the corner of a wrestling ring; pads cover the harness [turnbuckle] that hold [each] rope to the post.

Mike ["Doc"] Emrick uses [the term turnbuckle] metaphorically, saying "he sent him flying into the turnbuckle!" after a big hit, just like a wrestling announcer would.

~Susan