for those of us who don't speak legalese, what does this mean?
A grand jury is a special kind of jury where there's a prosecutor and no defence. The jury gets shown a bunch of evidence, in private, and decides if they think there's a crime, and what crime there is. If they do, then the grand jury writes up charges and the prosecutor presses them.
In this case, the interpretation is that the investigator feels there's at least something worth sharing with a grand jury, and that the case is far enough along that they need to impanel (recruit) one. This doesn't mean you'd expect charges in the next few months, or even at all, but it means that work is continuing.
The GJ also lets prosecutors subpoena people for testimony and expands the ability for the investigation to gather data.
One possibility is that Mueller will use the GJ to threaten charges against low-level actors to get them to cooperate and testify to give more information to the investigation.
