Businesses you don't understand -- Here's a great rule of thumb that works for beginners and expert investors alike. If you can't clearly explain what a company does and how it makes money in a sentence or two, don't invest in it. There are literally thousands of publicly traded companies to choose from, and you should be able to find plenty of opportunities in easy-to-understand businesses.
You might hear experienced investors talk about the concept of a "wide moat," especially if you're reading anything about Warren Buffett's investment style. Just as a wide moat around a castle makes it difficult for enemies to invade, a company should have a "wide moat," too: a sustainable competitive advantage that will prevent competitors from stealing that company's market share.
Dollar-cost average: This sounds complicated, but it’s not. Dollar-cost averaging means investing a set amount of money at regular intervals, such as once per week or month. That set amount buys more shares when the stock price goes down and fewer shares when it rises, but overall, it evens out the average price you pay. Some online brokerage firms let investors set up an automated investing schedule.