Mueller is known to be investigating Trump's former National Security Adviser Michael Flynn as well as participants in the Trump Tower secret meeting with emissaries from the Russian government and Vladimir Putin, which included Trump's own son and son-in-law.
In a Los Angeles Times article yesterday about the second grand jury (link here), a Cornell University law professor was quoted as saying:
"This suggests the investigation will end with indictments."
Indictments lead to automatic arrest, booking, arraignment, surrender of a passport, and mandatory court appearances. They also many times lead to incarceration until trial unless a substantial bond is posted.
An indictment almost certainly means prosecution will follow, since the prosecutor who seeks the indictment also makes the decision about proceeding to a criminal trial. Usually the only way to prevent trial is for the indicted individual to plead guilty to certain charges in exchange for others being dropped. Pleading guilty almost always means prison time.
If members of Trump's campaign, his staff, and/or his family are indicted, it will seriously constrain his presidency. And that would not be a terrible thing.