To answer your first question: "what does HSS stand for?" Well, if you're a guitarist...you know. HSS stands for "Humbucker Single-coil Single-coil". It's one of the two standard pick-up options on a Strat style guitar (Fender Stratocaster), and is one of the most versatile pick-up setups available, especially within the indie & surf rock world. The two Single-coil pickups, towards the front of the pick-up area (neck & middle), are a bit mellower and smoother and just have a lot of soul and character to them. The Humbucker, at the tail of the guitar (the bridge), has a higher output and, quite simply, has the ability to shred. However, some guitars have a button or switch that can "coil split" the Humbucker pick-up, turning it functionally into a Single-coil. Regarding the bridge (tail) position pick-up, let the Humbucker represent a 2+1 fin setup and the coil split Single-coil option represent a single fin.
Thanks for sticking with my (hopefully poetic) inner thoughts & brainwaves. Now let me explain the HSS in nonmetaphorical terms.
Simply put: the HSS is designed for a little bit of everything in the small-to-medium size & soft-to-almondy shaped wave world. It's not solely high performance based, and it's not solely soulful based. It's a beautiful meshing of the two in a single board.
Paddle power is a mainstay for the HSS; it was designed to be a great wave catcher. And along with the design concepts that create that (namely the relaxed nose rocker) also comes really nice inherent speed and some trimmy feel, allowing you to find that coveted highline, find trim, and lock in for the ride. The board is nicely walkable as well, primarily designed to be walked from the tail to two-thirds of the way up, though experienced surfers will find a few cheater-5s here and there.
Those soulful characteristics are balanced with some performance ones as well and what really sets the HSS apart from the other midlengths. The rails and tail of the board are performance based to allow for excellent responsiveness & agility off the tail and fins. The HSS is a swiss army knife of midlengths for those who want a healthy amount of speed & flow, but to always have that ability to crank a turn off the tail when a section presents itself.