These two peaks indicate that there are two values of metallicity which are more common than the rest. What this tells us then is that within our galaxy there are two main types of stars, ones with a metallicity of -.7 and ones with a metallicity of -1.5. What does this mean on a more general level?
If you think back to the physical meaning of metallicity you might remember that metallicity is an analog for the age of a star. Thus putting these two facts together one should come to the conclusion that our galaxy is composed of a group of young stars rougly the same age and a group of old stars roughly the same age.
Such a structure cannot be accidental and points to a physical property of our galaxy which causes it to form in such a way. The resons for this young/old structure will be discussed in much greater detail in further exercises, but for now it is only important to clearly see that such a structure exists, If you wish you can read all about what this structure tells us in the further information section.
Once you are happy with your can move on to further exercises.

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