An action in this context is something a character attempts to do, that has a risk of failure and where the consequences are of interest.
Volt uses two distinguishable 20-sided dice. We will assume that one die is black and that the other is white. An action is resolved by comparing a dice roll to a value; usually an attribute. If at least one die is below or equal to that value, then the action is successful.
Example: A character tries to patch up a wounded comrade using the attribute Medicine, value 10. The dice are rolled: 12 and 5. 5 is below 10: the action succeeds, the bleeding stops.
Tasks are made easier or harder by modifications. For instance, a particularly difficult task might incur a -3 penalty to the value. Similarly, an easy task could receive a +3 bonus, if a roll is needed at all. Bonuses and penalties are cumulative in theory. In practice, the GM considers the factors and just rules it.
If it’s relevant to determine how well a character succeeds, look at the highest successful die. The last example is a 5 Success. If no die is successful, the highest die rates the failure instead. A high successful roll is better than a low successful roll, and a high failed roll is less of a failure than a low failed roll.
The result is objective - not relative to an attribute or difficulty. That means “5 Success” is equally good regardless of the attribute value. Increasing an attribute unlocks higher successful results while preventing the most severe shortcomings. With attribute value 12, 13 is the worst failure you're still capable of.
When it is possible to add 20 to a die and still succeed, do it.
Example: With a +6 bonus, an already superhuman attribute value of 16 is now 22 for one roll. The dice show 13 and 2. The action is a 22 Success (2+20 is still a success).
Stealth, Melee and Ranged only function as modifiers or thresholds to other rolls (Perception and attacks, respectively). They are never used as independent values for resolving actions.
Roll the observer's Perception, minus the sneaking character's Stealth. Any penalties from distance or target size, as described under Ranged attacks, also apply to Perception rolls.