Airbanda
Google AoA! Generally the angle between the wings effective chord (the shortest distance between the leading and trailing edges of the wing) and the relative airflow. Measured by externally mounted probes which vary in function by manufacturer. Augmented on some aircraft with acceleration sensors/data. It can get very elaborate.
Fundamentally, low airspeed requires an increase in this angle to maintain level flight. To reduce speed further requires additional lift, such as is provided by flaps or slats.
At higher altitudes, air density is reduced and it becomes manifestly important to limit AoA and protect speed, not allowing it (speed) to decrease below that required to ensure sufficient airflow over the wing to maintain level, or in this case, ascending flight.
If you approach the AoA in any flight regime at which the speed becomes insufficient to maintain the required lift, the sensor should provide sufficient warning to reduce the angle or increase speed. Certain aircraft have stick push systems, also triggered by this sensor, to reduce the angle should other cues be missed by the pilot.