The market capitalization, or market cap, of a company is the total value of all outstanding shares of their stock. In other words, it's the number of shares multiplied by the current price for one share.
At the time of this writing AMD has 1 billion shares outstanding at 40 USD per share, hence their market cap is 40B USD. Market cap is a measure of a company's size: large-cap companies have a market cap over 10B USD, mid-cap have 2 to 10B USD, while small-cap have 300M to 2B USD market cap.
Investopedia's definition of market cap
Wikipedia's definition of market cap