It's a measure of how much they vary from a "flat" response. It might help you to visualize this by looking at the Gem's frequency response graph.
No speaker is perfect in this regard, and even if one were, it probably wouldn't be in a typical real-world room. One problem with such measurements is that they tell you little or nothing about how a speaker actually sounds, especially in a given listening room which can greatly affect the sound.
As for the concept of dB's, it's way too complex to describe here. Suffice to say it's a logarithmic measurement of sound pressure. For some context, 1dB is the smallest variation an average human can detect, 3dB is noticeable, and 10dB sounds twice (or half) as loud. For more, try this discussion (the sound files are useful aids) or Google away. A ton of information has been published on the subject.