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The chart shows the Palmer Drought Index for the currently selected state. The Palmer Drought Index is a measure of long term drought. It calculates soil moisture based on recent precipitation and temperature. 0 is considered a normal level of soil moisture, while increasingly negative numbers indicate progressively severe levels of drought. Positive numbers represent increasingly wet soil conditions. For full details see the Wikipedia entry.
Compare the Palmer Drought index to the actual rain fall level. California, for example, gets almost no rainfall in the summer. The chart below shows the amount of precipitation for the time period shown. Note that wet periods are often interspersed with drier periods. However, the net effect tends towards drier conditions.
The tendency towards less moisture is further exacerbated by temperatures that are greater than normal. So, even during the winter, the precipitation that does fall tends to be rain instead of snow. Snow levels are important to much of this area, as snow acts as a natural storage system that melts slow over the spring and summer. This tends to reduce the impact of the lack of rainfall that the summers often bring. The chart below shows the departure from normal temperature levels for the time period shown.