Resources are split into three categories: Focus, which determines how long the "slow-down" time distortion effect lasts while aiming your bow, Elf-Shot, which is your ranged ammunition, consumed when you fire arrows and perform Shadow Strike attacks, and Health, which determines how much damage you can sustain before dying.
The three weapon types are the bow, sword, and dagger. Improving one of these "Attributes" allows Talion to equip another Rune in that weapon. Each weapon can have up to five Runes. Each weapon tree also has a special ability that gives Talion the benefit of unlimited Executions (Storm of Urfael-Swords), unlimited Focus, Elf-Shot, and Fire Arrows (Flame of Azkar-Bow), or the ability to perform unlimited Stealth Kills (Shadow of Acharn-Dagger). You do not need to purchase all the Rune upgrades before purchasing one of these abilities, you merely need enough Mirian. These three skills are essentially "I win" skills, useful for tougher battles.