Once upon a time when digital didn’t exist, cameras used film, associated with the film you purchased was something called the ISO or ASA. This basically told you how sensitive to light the film you purchased was, you would see a number like 100, 200, 400, 800 and so on. The lower the number was the less sensitive the film was and how fine the grain was (noise). As you can probably guess having a low ISO film indoors was going to lead to underexposed photos and having a high one outside in bright sunlight would lead to overexposed photos.
Fast forward to today and now we have digital cameras ISO hasn’t actually changed that much. Instead of the film being sensitive to light, it is now your sensor. The lower the ISO the less sensitive it will be to light and in turn will have less noise the higher the ISO the more sensitive the sensor will be so in low light you can still get the shots you want but it will have more noise associated with the image which could, in turn, soften it.