The market value of all Bitcoin Cash (BCH) in circulation
Market Capitalization = (Price of 1.0 BTC) x (Total units of Bitcoin Cash (BCH) in circulation)
Similar to the way the Market Capitalization of a company reflects the perceived worth of its business, the Market Capitalization of Bitcoin Cash (BCH) reflects the perceived worth of Bitcoin Cash (BCH) as a value network. It's also an important metric for liquidity and utility of the network (for example, it would be difficult to perform billion dollar transactions at a Market Capitalization of $20 billion).
Fiat currencies do not have fixed and known supplies, allowing estimation as the only tool for Market Capitalization discovery. Gold has a Market Capitalization of approximately $7 trillion dollars.