Your camera ISO setting regulates how light sensitive the camera sensor is. When you are photographing in auto mode, the camera takes care of ISO settings for you.
Think of the sensor as a surface with light sensitive dots. The number and size of the dots varies with the ISO setting. A low ISO value means more but smaller dots, and vice versa. Smaller dots are less sensitive to light and bigger dots are more light sensitive. When the dots are bigger there will be room for fewer of them on the sensor. This will be visible in the photo as “grain”.
A low ISO value, like 50 or 100, makes the sensor light sensitivity low and there will be more but smaller light sensitive dots. When there is plenty of light, like daylight, a low ISO means sharp (not grainy) photos.
A high ISO value, like 1,600 or 3,200, makes the sensor light sensitivity high and there will be fewer but bigger light sensitive dots. In low light situations, like indoor in the evening, a high ISO means grainier but better exposed (not dark) photos.
Low ISO value = low light sensitivity, less grain, good for daylight conditions.
High ISO value = high light sensitivity, more grain, good for lowlight conditions.