Instead of buying a set amount of shares, you specify a dollar amount to invest – such as $50, $100, or $500 – so, if you invest $100 into a stock that is priced at $4.72, you would buy 21.1864 shares.
If this style of investing piques your interest, try the Automatic Investment Plan. It's geared toward dollar-based investing. It's great for people who want to invest a set amount of money each week, month, or year to steadily build up their portfolio.
If you'd rather buy a specific number of shares for a particular stock, you can use the Trade Now feature.