Ups and downs
Prices do not move in a straight line. How much they swing up and down is what investors usually call risk.
In plain words
A savings account barely moves. A broad fund of shares can move a few percent in a week. A single company's share can move that much in a day.
Why it matters
Big swings are hard to sit through, and people often sell at the bottom. The real danger is a fall right when you need the money.
Time changes it
Over one year, shares can fall by a third or more. Over many years, broad markets have usually ended higher, but there is no promise: the past is not a guarantee.
Putting a number on it
Volatility (the size of the swings), drawdown (the deepest fall) and the Sharpe ratio (reward for each swing) are ways to measure it. They are on the Ups and downs shelf.
Spreading helps
Owning many different things makes the swings smaller, because they do not all fall at once.