You have buildings that produce culture in your city. This is your Total Culture. 129482
You have buildings in your city that need culture. This is your Required Culture. 73869
Total Culture - Required Culture = Available (unused) Culture.
129482 - 73869 = 55613
Every Culture Building you have in your City goes towards your Total Culture, but then Culture is deducted from this when you make certain upgrades or build certain buildings. The Culture left over from this is what provides the Boost for your Supplies and Coins. The more unused Culture, the more Bonus!