When most people think of a jury, they think of a group of people reaching a verdict about someone's innocence or guilt. While this is true for trial juries, it's very important to note that this is not the function of a grand jury.
Instead, a grand jury is aimed at deciding if the right amount of evidence exists to go forward with a trial. If the jury determines that enough evidence doesn't exist, the case never even goes before a trial judge. If they do think there is enough evidence, then they give out an indictment. This does not mean that the person is guilty in any way, but just that there is sufficient evidence to find out.
The point of the grand jury, then, is to keep cases from going to court if there is so little evidence that the cases are pointless.
Often, a grand jury is just going to be used for a serious federal crime, like murder. For instance, if there is a shooting and witnesses claim that a certain person did it, a grand jury may be used to determine if there is any actual physical evidence to support this claim. If this tangible evidence can't be produced, the jury may not indict the person, no matter how adamantly witnesses insist that the person is guilty.
What this means is that the grand jury looks at the evidence a bit differently than a trial jury. It just needs to see if the evidence exists, not if the evidence means the person is guilty.
If you're facing federal charges in Florida, you need to know about every step in this process.
Source: Federal Judicial Center, "How Cases Move Through Federal Courts -- Criminal Cases -- Indictment or information," accessed June 19, 2015