Market cap is just a broad way of letting you know how much the total amount of coins are worth.
If one coin has 10,000,000 in supply and each coin costs $1 then the market cap is $10,000,000, as the price of the coin increases the market cap increases.
So, coins with higher supplies like Ripple need a much higher market cap to be worth more indiviually than those with lower market caps because the supply for Ripple is 38 billion while something like Ethereum has 95 million in supply, $73 billion market cap yet one individual Eth is worth 380x more than 1 XRP even though Ripple has a higher market cap.