The total market value of the supply in circulation ['Market cap' = 'Supply in circulation' * 'price'].
You can answer the question "what is more valuable 1 Bitcoin or 1 Ethereum?" by comparing its price, but to answer the question "which cryptocurrency has more value in total Bitcoin or Ethereum?" you need the market capitalization. The market capitalization of a cryptocurrency takes the current supply in circulation into account. There are only 17 million BTC in circulation but 98 million ETH. If BTC and ETH both had the price of 1 $ the market capitalization of Ethereum would be 98 million $, but the market capitalization of Bitcoin would be only 21 million $.