Below the health/stamina meter, there is a knife that flashes yellow most of the time. Even below that, there's a red meter that seems to change functionality depending on what weapon you're using. When I tried out the Katana, it would fill up as I attacked and then deplete over time. Now that I'm using two swords, it doesn't seem to do anything at all.
What is that meter for? What does it indicate?
The location of that meter is reserved weapons with a unique functionality which require an additional bar. It's functionality varies from weapon to weapon, and doesn't even exist for some weapons. Other weapons will have something else in it's place which isn't a meter at all.
The weapons which make use of that spot are:
Dual Sword: Demon Gauge. To be honest, I'm not that familiar with the Dual Swords, so I'd suggest looking into this video tutorial for more information. The basic idea is that Dual Swords can enter something called Demonization mode, which gives you additional combos and improves you attacks. When you hit monsters in demon mode the meter fills up, and so long as the meter isn't empty you'll do extra damage and attack faster when not in demon mode.
Long Swords: Spirit Gauge. Once again, I'm not that familiar with the Long Sword, so I'll refer you to here for more information. Attacks with a Long Sword fill up the Spirit Gauge. Once the Spirit Gauge is full, Spirit Attacks (made with the R button) no longer cost Spirit. A Spirit Combo performed while the Spirit Gauge is full will have a finishing attack called the Spirit Roundslash. Your hunter will automatically sheathe his/her weapon after performing a Roundslash and receive a minor damage boost. Landing the Spirit Roundslash will also increase the grade of the Spirit Gauge (white -> yellow -> red). Higher grades of the Spirit Gauge will provide higher damage boosts while degrading quicker.
Bowguns / Gunlance: These weapons require ammunition, and they use this spot to displays how much ammo is remaining in the weapons clip.
Switch Axe: Phial meter. The Switch Axe has two modes, Axe and Sword, and every Switch Axe has a phial. Axe mode generates phial energy, and Sword mode consumes it. The functionality of the phial varies from Axe to axe, but are only active when in sword mode. For example, a Power Phial will boost the Raw (physical) damage you do. The phial meter displays how much phial energy you have left.
Hunting Horn: Note meter. The Hunting Horn produces a musical note with every attack, which are displayed in the note meter. Some sequences of notes allow the Hunting Horn to play a song by pressing the R button, which gives the the user and nearby allies a temporary buff. Their are a variety of buffs, including healing, attack boosts, and giving users infinite stamina.
