ISO simply stands for the International Organization of Standardization, which is the main governing body that standardizes sensitivity ratings for camera sensors (among many other things). … With both analog and digital cameras, ISO refers to the same thing: the light sensitivity of either the film or imaging sensor.
Here’s a good stock setting for outdoor headshots, landscapes: set Manual mode, Auto ISO with shutter speed at 1/60 sec and the aperture at its widest setting, such as f/4. use a similar setting but with ISO 100 and work your way up until you get the results your looking for. Take test shots to work out the best pairing with the aperture and shutter speeds.
I always set the ISO last to keep my photos from getting too noisy and grainy looking in the shadows.
Recommend this video for a more entertaining way to enjoy learning what ISO means and is.
ISO: The Ultimate Guide