Market Capitalization or "Market Cap" is the value the market puts on a publicly-traded company. You can find market cap by multiplying the total shares outstanding by the purchase price.
Why It's Important
Market Cap is used to value and gauge the public opinion of a company. Amazon's market cap is currently about $856 billion (as of 10/11/19). That's huge! Their stock price is $1,731.92. You would think that the higher the stock price, the higher the market cap, but that is not always the case.
In comparison, Apple's market cap is $1 trillion, but its stock price is $236.21 (as of 10/11/19). This information tells me that Apple has more stocks outstanding than Amazon. Keep in mind that the market determines the market price, and every share a company sells is some ownership of the company. When you are researching what stocks to invest in, be sure to take a look at their market cap as part of your valuation.