Availability refers to how often the creature is seen in the pet trade.
VC or Very Common - This means that this particular fish is a staple of pet stores, as well as regular stores with a small aquatic section, and breeds easily in captivity and relatively hardy.
C or Common - This means it will be available to purchase from all leading aquatic stores, but less likely to be seen in regular smaller pet stores with a small aquatic section.
U or Uncommon - This means that this fish will only be stocked in leading aquatic stores with good wholesale links or specialist aquatic stores, i.e. stores that specialize in Marine or specialize in African Cichlids. It may also only be available as a wild caught specimen or only occasionally be bred in captivity. These fish are probably best ordered from aquatic stores or online and are unlikely to be available to spontaneous walk-in customers.
R or Rare - This means these fish are very rarely available in the pet trade, either they're threatened in the wild, do not breed in captivity at all, or possibly need such specialist care they are left in the wild and not risked to be brought in through wholesalers.