Real basic it works off a compass heading, when you set it it track that heading.
Which doesn’t work perfect in a cross wind or current area, because the boat can slide off the line, but still be heading in the same direction (does that make sense?)
Truth be told there has been some issues with it, sometimes they spin the motor in circles and get a little funny, I’ve been able to correct those issues in the past with either a recalibration or a new compass.
To me it’s one of the best options on my boat, I love the auto pilot, it helps me maintain better boat control in any condition, on a lake on a normal day, I works perfect, on windy days, and on the river it take a bit of a helping hand to keep perfect boat controlm, sometimes I have to correct it a little to hold a spot on line a little better, but way, way less involved than any regular trolling motor.
Personally every trolling motor I buy has it, I love the option myself, and of course it doesn’t have to be used, you can still run the motor with the AP off, and it runs just like a normal trolling motor. An expenisve option, but it’s worth every single penny to me. (AS is the Universal sonar)