Volts

Volts are small white pickups dropped by Enemies when they are destroyed.

Boss phases also drop weapon power, either gradually until they are destroyed, all at once when they are destroyed, or both (depending on the boss)

Collecting them allows you to upgrade your primary weapon up to a maximum of Level 3 (starting at Level 0).

Naming
"Volts" is the name used by game files and by the developers. They are also referred to as "Energy" in the tutorial.