2.1 Basic rules

Rolls
Whenever you try to do something that takes effort, you’ll probably have to make a check for it. When you make a check, the DM sets the DC (difficulty class) of the action you’re attempting. You then roll d20 and add the appropriate stat. If the result equals or exceeds the DC, the action succeeds. Attacks resemble checks, and are discussed in 2.1 Combat.

Opposed checks: An opposed check means that two creatures make a check and then compare results to see who won. If they both roll the same value, roll again until a winner emerges.

Taking 10: If you're not in combat or otherwise threatened or rushed, you can take 10 on a check, meaning you treat the die roll as 10 without rolling. This is useful for easy checks that still have a small chance of failure.

Advantage and disadvantage
When making an attack or check under hindering circumstances, you have disadvantage, meaning you roll two d20s and must use the lower roll. If the circumstances work in your favour, you have advantage, meaning you roll two d20s and use the higher roll. If both advantage and disadvantage apply, they cancel each other out and you roll normally. If more than one case of the same applies, it stacks, causing you to roll three or more dice and take the highest/lowest one. This is called double/triple (dis)advantage.

Injury and recovery
If your HP falls to 0 or below (in which case it becomes a negative number), you're knocked unconscious and are dying. While dying, you lose 4 HP (ignoring DR) at the start of each of your turns. If your negative HP is equal to or larger than half your max HP (so -15 if your max HP is 30), you die.

On their turn, an ally can spend a full-round action to stabilize you. When stabilized, you remain unconscious, but no longer lose HP automatically. You also become stabilized if you receive any magical healing. Taking any damage when below 1 HP causes you to resume dying. You regain consciousness when your HP is healed back to 1 or higher.

You can regain HP by resting. To take a rest, you must spend eight hours in a safe location while undertaking no strenuous activity. Get a night’s sleep, basically. After a rest, you regain 50% of your max HP and refill your energy pool.

Miscellaneous rules
These are some mechanical rules that are important to note.

Rounding
Whenever a formula results in a fraction, you always round down.

Effect durations
Effects with a set duration always end at the start of the turn of the creature who caused the effect. Effects with a duration of one round end at the start of the caster’s next turn, those with a duration of two end at the start of the turn after that, etc.