Got it, but I still don't understand what
"And if you want some leverage, borrow against your stock with a credit line." means. How does this increase leverage?
By borrowing against your securities you have more cash to invest. You've put a multiplier on the amount you can invest. That's leverage. However, you have to pay back the loan with fees and interest. This increases risk because you could end up owing money rather than just having none if the securities crash.