This doesn't guarantee an indictment, but it's a necessary step in getting an indictment. The DOJ has to be involved in order to convene a grand jury - and only a grand jury can indict - so once the DOJ gets involved with the FBI, you at least know an investigation has reached the point that they're finding enough to compel them to prepare for that next step.
In other words, all the claims of Hillary and her campaign that there's nothing there are worth about as much as most everything else that comes out of Hillary's mouth: