You are looking at the Fletcher-Munson equal loudness curves.
The point, which no other answers have explained yet, is that this is a graph of how the human ear responds to sounds and loudness. The human ear and brain do not perceive a linear increase in sound pressure levels in a linear fashion, and the human ear and brain's perception of the volume of a given area of the frequency spectrum, in relation to all other areas, is different at different sound pressure levels.
When I studied recording studio engineering in college, this graph was used to impress upon us that music recordings must be made, mixed and mastered with the human engineer listening at a moderately loud volume level (80 decibels or so), because if one were to listen at too low a level, one's ears would not give an accurate reflection of how the music would sound to a listener playing it back on a car radio, stereo system, or what have you. For example, if recording and mixing were done with the engineer listening to a very low volume, one would mix in too much bass to over-compensate for a perceived lack of bass in the recording. Then when consumers played the recording back on their stereo systems, there would be much too much bass coming out of their stereo system.