Complex sentence: Companies raise billions of dollars through the issuance of securities in the primary market--conversely, trillions of dollars are made and lost through the trading of those same securities in the secondary market.
Split into simpler sentences: Companies raise billions of dollars through the original issuance of securities in what is known as the primary market. Conversely, trillions of dollars are made and lost through the subsequent trading of those same securities in the secondary market.