Risk is the gamble we take that whatever we invest in a venture may not pay off. A sound rule is never to risk what you are not prepared to lose.
The costs and consequences of failure are downside, as opposed to the positive benefits of upside if all works out.
If one starts form the position that, as soon as an investment is made, it is lost until proven otherwise, it is easier to focus on maximising upside rather than scaling back ambition to minimise downside.