The line of thresholds for each frequency (and each ear) create a sort of barrier.

Above the barrier are the sound levels for each pitch at which the individual did not respond. Thus, they did not detect these tones when presented (or detect them consistently).

Below the barrier are the sound levels for each pitch that the person could detect. This means that this person should be able to detect sounds that are presented at the levels within the shaded area.

However, this does not mean that the sounds are "clear" to the person, or that speech that could fall within this region would be understood. These are just the quietest levels at which this individual could hear each frequency on this particular day.

Thresholds can change from day to day, depending on if wax is blocking the outer ear canal, if the person has an ear infection, etc. Many factors can influence thresholds. And thresholds can certainly vary from frequency to frequency.