Market Capitalization is the total value of all outstanding shares, tokens, or units available for a given traded item. In cryptographic tokens market cap refers to the total supply of coins available, say 21 Million times the current price. If the current price was $1, the market cap would be 21 Million.
Similarly Metal's market cap is the outstanding Metal times the current price. It can be denominated various ways, like USD, Euros, Bitcoin or Ether.
To check the Market Cap of Metal you can visit either one of these websites.