Each time the party gets through an encounter with something that would place at least one of them in peril (i.e. something that would deal damage or otherwise be harmful, whether or not anyone was actually harmed - whether it's a trap, a combat encounter, a deadly puzzle, etc.) they gain a point of Adrenaline. Characters can accumulate a maximum of 3 points of Adrenaline. Taking a short rest reduces your Adrenaline by 1 point, and taking a long rest reduces it to zero.
So, what does Adrenaline do? Very simple: For almost all purposes, your current Adrenaline score is added to your Proficiency bonus. So it boosts your attack rolls, your proficient saving throws and ability checks, your spell attack bonus and your spell save DC. The only thing it doesn't boost is the number of uses of an ability whose number of uses is determined by your Proficiency bonus.
This is obviously pretty powerful. A party with 3 Adrenaline can punch significantly above their weight. But they lose Adrenaline every time they rest to replenish their resources.