The marketcap is the total number of coins in circulation multiplied by the price of each coin. So Bitcoin's marketcap is 16,093,600 x $908.44 = $14,620,118,265.

The volume for Bitcoin is $119,630,000 for the last 24 hours. So there's tons of liquidity there. If you want to sell your Bitcoin, you will probably have tons of buyers buying at that price $908.44.

The volume is important cuz you can look at a coin or token and buy low and then go to sell high for profit. However, if there is no liquidity, you may not be able to sell it at that top price because there are not enough people buying at that top listed price.

Look at all the ones that have higher prices than Bitcoin: 23 skidoo, satoshi card, bitgold, bitbtc. All higher prices than Bitcoin currently, all low volume. The chances of finding buyers to sell at those prices are much slimmer.

Hope that helped explain it some :)