In money, standard deviation may mean the risk that a price will go up or down (stocks, bonds, property, etc.). It can also mean the risk that a group of prices will go up or down (actively managed mutual funds, index mutual funds, or ETFs). Risk is one reason to make decisions about what to buy. Risk is a number people can use to know how much money they may earn or lose. As risk gets larger, the return on an investment can be more than expected (the "plus" standard deviation). However, an investment can also lose more money than expected (the "minus" standard deviation). 