The light that passes through the extreme sides of the lens is evaluated by two image sensors
Based on how the light reaches the image sensors, the AF system can determine if an object is front or back focused and by how much
The AF system then instructs the lens to adjust its focus
The above is repeated as many times as needed until perfect focus is achieved. If focus cannot be achieved, the lens resets and starts reacquiring focus, resulting in focus “hunting”
Once perfect focus is achieved, the AF system sends a confirmation that the object is in focus (a green dot inside the viewfinder, a beep, etc)