Circulating Supply × Current Price.
The market cap is derive by multiplying the current market value of an asset by it's circulating supply.
Market Cap is one way to evaluate how much support a crypto asset has, but it's just one of many factors to consider.
As market cap is derived from the current price, sudden large price changes cause very large changes to the market cap.
That is to say if everyone sold off their assets or bought an asset together, the market cap can swing up or down much faster than expected.
When looking at whether a coin is "cheap" or not, it's important to consider the market cap. A coin that is worth $0.50 but has a market cap of $20 billion can be valued the same as a coin worth $500 with a market cap of $20 billion, with them having a different the circulating supply.
Just because a coin is cheap doesn't make it a good investment, and just because it's expensive doesn't make it a bad investment.
* All terms and definitions may update as the Cryptionary improves.