When discussing ISO settings, using the 'slowest possible' means:

The term 'slowest possible' when discussing ISO settings refers to using the minimum ISO that allows for proper exposure. A lower ISO setting, such as 100 or 200, is typically associated with less sensitivity to light, which means it is better for shooting in well-lit conditions or when a slower shutter speed is acceptable. This allows for improved image quality with reduced noise in the final image.

In situations where you have sufficient light, opting for the slowest ISO helps to maintain the highest image quality by minimizing graininess and preserving detail. As a result, photographers often prefer using the lowest ISO setting that still achieves the desired exposure, especially in landscape or portrait photography where image clarity is paramount.

Higher ISO settings tend to introduce more noise and can adversely affect image quality, which is why using the slowest ISO that provides the proper exposure is often the preferred approach.