The masker is a sound that 'hides' all other signals around it by dominating that auditory band in human hearing. In that graph, it is the tone that masks the darker bar at around 400 Hz. Every frequency sound with an amplitude that lay below the solid line would be rendered inaudible by the masker.
Here is a cruder graph that also illustrates this effect:
We have this phenomenon to thank for us humans being so insensitive to harmonic distortion. As you can see, most masking occurs at higher frequencies than the masker although lower frequencies can be masked to if the masker has sufficient amplitude.