An indictment just means that a person was charged with a crime. The defendant is not guilty, but he was accused of committing the crime.
One special characteristic of the United States is that in federal courts and often in state courts a “grand jury” indicts the defendant. A grand jury is a large jury, 16-23 persons, which reviews the evidence presented by the prosecutor and determines whether there is at least enough evidence to accuse the defendant of committing the crime. Grand juries are intended to, among other things, act as a shield against meritless prosecutions. Not all states use the grand jury system and some states only use grand juries for certain crimes.
If the grand jury agrees that there is enough evidence the foreperson of the grand jury will sign the document with the accusations against the defendant. We would say that the grand jury has indicted the defendant.
Below is a short video on grand juries.