The market cap of a digital currency is the total coin supply multiplied by the coin's market price. The market cap is of course an intersting metric it is not the only one to consider when ranking cryptos.
Why the market cap tells not the whole truth
There are many coins where only a small fraction of the total coin supply is actually available to trade. This is often a problem with premined coins.
A famous example is Ripple's XRP token of which the company holds the most. For such coins the market cap is a missleading metric.
What to consider besides market cap
A very important factor is the liquidity of a coin. If you sell a large amount of Bitcoin this will most likely not affect the price. But if you sell a large amount of a smaller coin such as Gridcoin this will lead to a great price decay. So if you want to trade you should not only consider the market cap but also the liquidity of a coin!
The liquidity is just the amount of coins that are traded in a certain time. A higher trade volume is better because your trades are more likely not to affect the price.