ISO is not too hard to understand: higher ISO values tend to have less detail than lower ISO values. This lack of detail is manifested in noise: hard edges in a noisy image are not so hard. Also, a solid surface will appear to be speckled.
Modern cameras, especially ones made in 2014 and later, boast extremely low noise for very sensitive sensors. Compared to the film days, when shooting ISO 400 film was considered extreme, one can usually get away with shooting at ISO 3200, even 6400 and above, on many cameras.