What's borsch? What symbolically significant about this being Zaroff's meal when we first meet him?

Borscht (spelled 'borsch' in the story) is a sour soup made from beetroots. It's an ethnic cuisine common to many Eastern European countries. 
Borscht is introduced in the story in the following passage, in which Rainsford is speaking with General Zaroff and Ivan:

They were eating borsch, the rich, red soup with whipped cream so dear to Russian palates. Half apologetically General Zaroff said, "We do our best to preserve the amenities of civilization here. Please forgive any lapses. We are well off the beaten track, you know. Do you think the champagne has suffered from its long ocean trip?"

Zaroff's borscht serves two purposes in this role. The first is easy to spot, since Zaroff explains it himself. It's a sign of civilization--an ironic sign, since Zaroff is a killer who hunts and murders humans. 
The second is subtler in the reading, but it's unmistakable if you look at borscht for yourself. Dyed crimson by the beetroots that give it its characteristic sour flavor, a bowl of borscht looks much like a bowl of blood. Zaroff dining on thick red liquid symbolizes his bloodthirsty nature as a hunter of men.