What is Auto ISO?
Most modern DSLR and Mirrorless cameras have an “Auto ISO” setting which tells the camera to choose a suitable value. This is helpful when the light is quickly changing so that you don't have to keep on adjusting.
Here's a detailed guide on the different applications of Auto ISO:
https://digital-photography-school.com/a-beginners-guide-to-auto-iso-and-other-camera-modes/