All actions will be given a difficulty which will dictate what the 2d6 roll actually means. For simple actions, a 5 or above is an automatic success. Simple actions are things like opening doors, reloading your weapon (provided it's not some arcane piece of alien technology), asking a friendly NPC for information, buying something from a shop, throwing a grenade a few meters, whatever. This means you have an 83.33% chance of doing it successfully. Even if you do fail a simple action, chances are the outcome won't be catastrophic -- unless you roll a 2. If you roll a 2, then I roll 1d6 to determine the consequences of your critical failure.
Moderate actions are things that require a bit more finesse. Lockpicking, reverse engineering something, firing your weapon at an enemy (and hitting), sneaking around in the dark, persuading a neutral NPC to do something within reason. 7 or above is a success here, with the rewards for success becoming greater the higher the number (unless you roll a 12, which requires a 1d6 roll for the consequences of your overshot). So you have a 58.34% chance of successfully accomplishing one of these things, before any bonuses. If you fail, the outcome is also rarely catastrophic, unless it's a 2 or 3. You will earn skill points regardless of the outcome, but success earns you more.
Difficult actions are things that border on impossible for your character. This includes hacking computer systems with no relevant skills, assembling complex technology without blueprints, convincing a hostile NPC to do anything except kill you. A 9 or above is a success for one of these actions, so you have a 27.78% chance of success. Even if you fail, you will gain skillpoints, so you might have a better chance next time. If you fail a difficult action, chances are you'll break something or enrage an NPC.
Impossible actions are just that: physically impossible actions. Not even things that are unlikely or almost impossible. These are actions so far out of your character's element he/she wouldn't even have an idea where to begin. This includes going against established game rules, spontaneously wishing stuff into existence, whatever. Anything but a 12 is an automatic failure. Even if you roll a 12, I'll then roll 1d6 to determine the effects of your action, using standard RtD rules. Failures at this difficulty will be catastrophic, and you will not gain any skill points. These are actions best avoided, unless you're 'strategically' trying to kill yourself and your friends.