Within 10 days of the initial appearance for in-custody defendants, and within 20 days of initial appearance for out-of-custody defendants, a defendant is entitled to a preliminary hearing or arraignment. See Fed. R. Crim. Pro. 5.1. There are federal grand juries sitting at all times in the District of Guam, so a defendant will be arraigned on an indictment at the arraignment hearing, instead of having a preliminary hearing.
An indictment is a formal charging document that contains the federal charges faced by the defendant. It is reviewed by a grand jury, and if there is sufficient evidence to force the defendant to face the charges the grand jury signs off on the indictment (or “returns the indictment.”) There can be many indictments brought in one case – later indictments are called “superceding indictments.”