What does "Dark Background " mean TT terminology

With respect to TTs, it means that there is very little surface noise when the music is not playing. Such as between tracks. There can be a great deal of difference between the amount of surface noise that you hear from one table to the next even with the same record. One of the many reasons I chose Galibier. Teres tables are also very good at dark backgound.

Well, F#$%k, Why can't they just say very quiet in between tracks. Geeeeeeez!

I agree w/ Dan but would like to add lack of TT rumble and resonances as contributing to a quiet background. I also think that (everything else being equal -- clean record, etc.) the cartridge (more specifically the stylus) has a lot to do with the amount of surface noise you hear or don't hear.
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It's more than just quietness between tracks. Many components always produce a certain amount of background noise, like preamps and amplifiers as well. There is something called a noise-floor, the point of audibility for this type of noise. Record players have a noise-floor too. The lower the noise-floor, the more actual musical information surfaces, as the more delicate less audible details, resonances and so forth, get wiped out by a more prominent/higher noise-floor. A turntable's noise floor is determined not just by surface noise, but also by environmental noise coming across the air or through the feet/stand, noise from the motor, from the belt-and-pulley or idler-wheel or whatever system, and from its own bearing and plinth design's ability to cope with these various sources of noise. A turntable with an especially black background reduces the noise floor to a greater extent than one with less black backgrounds, which translates into more detail and various other types of information, which means it is audible (by not being there) during music as well. So "quiet between tracks" doesn't quite cover it. Hope this helps!!

I would add to Johnnatais's comments that a blacker background with a TT or CDP usually implies more color and constrast in the music itself. Critics sometimes use the opposite term "whitishness", to describe a lack of constrast or color. Of course all these terms are analogies...