RS3 combat basics: Revolution, adrenaline and abilities
How RS3 combat actually works: the Revolution system, adrenaline, and the four tiers of ability, condensed from the wiki and checked, not remembered.
Revolution does the clicking for you
Revolution is a combat mode, released 24 February 2014, that automatically triggers the first available, compatible ability on your action bar for you. Toggle it from the Combat Settings interface, or by right-clicking the gear icon on the action bar.
It can access any number of slots on the bar, so a well-built Revolution bar already uses your abilities in a sensible order without you pressing a key for each one. You can still trigger any ability yourself with its hotkey at any time, Revolution or not.
Adrenaline is what lets you use the stronger abilities
Adrenaline is shown as a percentage next to your life points on the action bar. You build it mainly by using basic abilities; threshold abilities and some enhanced abilities need adrenaline to use, and ultimate abilities need a large amount of it.
Stand out of combat for 10 seconds and adrenaline starts draining 5% every 2 ticks until it hits zero, unless a specific relic power, buff or urn is active. Logging out, hopping worlds, or starting a new fight with another player in the Wilderness resets it to zero outright.
Clicking the adrenaline bar itself activates your equipped weapon's Weapon Special Attack, if it has one; these generally sit between a threshold and an ultimate ability in strength.
The four tiers of ability
Basic abilities deal moderate damage, often with a secondary effect like a short stun, and are what actually builds your adrenaline.
Threshold abilities and some enhanced abilities cost adrenaline to use and hit harder, with stronger effects than basic abilities.
Ultimate abilities cost a large amount of adrenaline for a correspondingly large hit or a very strong effect.
If you use no ability at all, your character performs a plain basic attack instead, so an empty action bar still fights, just far less effectively.
Gear and skill requirements
Every ability has its own requirements, visible by hovering over it: a minimum skill level (temporary boosts like overload potions do not count toward this), a gear requirement (a two-handed weapon, matching dual-wielded weapons, or a shield, depending on the ability), and sometimes an unlock requirement from a quest or achievement.
This is why a RS3 loadout is built around a specific weapon setup, not just the highest-level gear you own: the wrong weapon type locks you out of abilities your action bar expects to use.