With fractional shares, you are able to buy a decimal amount of a share, meaning you no longer need to buy shares in multiples. This allows you to buy shares either in a dollar value or in number of shares.
When executing an order in shares, the order will buy the number of shares selected when the order is placed. (e.g., Placing an order to buy 2 shares of a company priced at $4 at the moment of execution, will cost you $8 to execute)
When executing an order in dollar amount, the order will buy as many shares as possible with the $ amount selected. (e.g., Placing an order in $ at an amount of $10 to buy shares of a company priced at 2$ at the moment of execution, will buy you 5 shares of that order). Note: $ orders are only available for fractional shares.