power-profiles-daemon offers to modify system behaviour based
upon user-selected power profiles. There are 3 different power
profiles, a "balanced" default mode, a "power-saver" mode, as
well as a "performance" mode. The first 2 of those are available
on every system. The "performance" mode is only available on select
systems and is implemented by different "drivers" based on the
system or systems it targets.
In addition to those 2 or 3 modes (depending on the system),
"actions" can be hooked up to change the behaviour of a particular
device. For example, this can be used to disable the fast-charging
for some USB devices when in power-saver mode.
GNOME's Settings and shell both include interfaces to select the
current mode, but they are also expected to adjust the behaviour
of the desktop depending on the mode, such as turning the screen
off after inaction more aggressively when in power-saver mode.