|Market cap is short for Market Capitalization. It's the total value of a coin calculated by multiplying the current price of the coin by the number of coins in existence.|
The term originated from the stock market where the calculation was the share price multiplied by the number of outstanding shares.
Bitcoin's market cap at the time of this writing is it's price of $58,088 * 18,876,555 = $1,096,501,326,840 (stats via https://bitinfocharts.com/bitcoin/)
There are many places to get this data, but I use https://CoinMarketCap.com