Plain English Meaning
The knob or input that lets the pilot tell the altimeter what today's air pressure is, so the altitude reading is accurate.
Full Definition
A control on an altimeter or air data computer that lets the pilot enter the current local sea-level pressure setting (in inches of mercury or hectopascals) so the instrument displays the correct altitude for the surrounding atmospheric pressure.
Context Anchor
Seen on electronic flight displays, often as a BARO setting, when the pilot enters or updates the altimeter setting before and during flight.