ADCS stands for attitude determination and control system. Let’s break this down even further. Attitude is the 3D-orientation of the spacecraft in a specific reference frame (a reference frame is a origin and set of 3 axis following right hand rule).
Common reference frames
Earth Centred Inertial (ECI) - fixed in space, does not consider earth’s rotation
Earth Centred, Earth Fixed (ECEF) - does rotate with earth’s rotation, therefore the axis always pass through the same locations on earth
Watch this clip to see the difference between the two: https://courses.ansys.com/index.php/courses/introduction-to-orbital-elements/lessons/coordinate-frames-epoch-lesson-3/
The attitude determination part identifies your satellites current orientation in space. To do this it uses the space environment and estimates your location.
List of Common Sensors (good overview: https://www.youtube.com/watch?v=p0iarTNt6tE)
Magnetometer - measures the magnitude and direction of the magnetic field, inexpensive but incomplete sensor as it doesn’t tell you the satellites orientation along that line. This means you have only two of three components of attitude.
Star Tracker - takes a picture of the sky and based on the orientation of the certain stars it will try to pattern match based on a catelog of star patterns. It requires a lot of CPU and power to pattern match and is heavy, but provides extreme accuracy. (for more detail: https://www.youtube.com/watch?v=hA1LsvgV2UY)
Sun Sensors - measures the direction and magnitude of the sun, sensitive to albedo (light reflection off of other objects)