I'm gonna go ahead and be "that guy" and ask what all this means. Does this mean there's enough evidence to charge him? Not familiar with this kind of stuff at all.
The short answer is that now A.J. is officially a defendant in a rape case. He is charged with a crime. (It seems there is some conflicting information as to whom was indicted, so let's just speak generally)
The long answer is that grand juries are intended to be a procedural safe guard that would stop the government from pursuing cases with no merit. Our forefathers knew that there was a history of the government prosecuting people in an effort to intimidate or harass. In theory the grand jury system is great (the U.S. is the only country that uses such a system, to my knowledge). In practice, they have been turned into something that is no more than a procedural rubber stamp.
Basically, at least in TN, the prosecutor takes all of his evidence, and even his witnesses, and puts on a case for the grand jury. It is a dog and pony show. The jurors don't hear anything the prosecutor doesn't want them to hear, and the defendant doesn't even have the ability to appear to defend himself. So, after the evidence is put on, the grand jury decides if there is enough evidence to continue with formal charges and therefore decides if a case should go to trial. The answer is usually a resounding yes as there is no other side of the story, there aren't even evidentiary safeguards (hearsay is allowed) etc.
Should the grand jury refuse to indict, that is typically a sign that either the prosecutor didn't want to indict (thus used the GJ as a shield against the public who elected him), or there really is very little evidence to proceed.
In all other cases, which is the majority, the GJ will return a true bill and the case will proceed towards a trial. Where it gets really interesting is when the prosecutor starts offering plea deals. The prosecutor might know that this case will lose in open court so they offer to plea it down to a misdemeanor (similarly the defendant's lawyer might know the case is a loser, and offer a plea to keep it out of open court as well). If the prosecutor offers no plea, that is generally a sign that they feel they have a great case and a mandate to make the defendant an example.
If no plea is reached, the case eventually goes to trial and who knows how it will turn out. It is important to know that the right to a speedy trial is one that must be asserted by the defendant. Usually a defendant has more reasons to allow the case to drag along, especially if they post bail, than they do to want a speedy trial (which could end with them going to prison).
That's just a general overview, and I hope it helped.
I believe Mark Twain said that a trial is where 12 jurors spend days deciding who has the best lawyer.