When the outcome of something is in doubt, you make a D20 Test: roll a twenty-sided die, add every modifier that applies, and compare the total to a target number. Meet the target or beat it and you succeed.
There are exactly three of them, and the difference is only ever who started it and what you are aiming at.
| Test | You make it when | You are trying to beat |
|---|---|---|
| Ability check | You attempt something uncertain | A Difficulty Class the DM sets |
| Saving throw | Something is done to you and you resist | A DC set by whatever is doing it |
| Attack roll | You try to hit something | The target's Armor Class |
This matters more than it looks. Anything in the game that says it affects "D20 Tests" affects all three at once — so a feature that gives you a bonus to D20 Tests is helping you hit, resist and attempt, not just one of them.
For a check or a save, the DM picks a Difficulty Class — the number to reach. The scale runs in steps of five:
| DC | How hard |
|---|---|
| 5 | Very easy |
| 10 | Easy |
| 15 | Medium |
| 20 | Hard |
| 25 | Very hard |
| 30 | Nearly impossible |
For an attack, nobody picks anything: the target number is the target's Armor Class, which is a property of the creature rather than a judgement call.
When circumstances are strongly in your favour you roll with Advantage: roll two d20s and use the higher. Against you, it is Disadvantage — two d20s, use the lower.
Two rules keep this from spiralling. They do not stack: three sources of advantage are still just two dice. And they cancel out entirely — if you have both, from however many sources, you roll one die as normal. Counting up how many of each you have is never necessary; you only ever ask whether you have any of each.
Heroic Inspiration lets you reroll the die outright. You must use the new roll, so spending it on a 9 is a real gamble rather than a free upgrade.