Special Investigator in Trump/Russia Collusion has now hired a grand jury to assist him, stepping up his investigation, suggesting he has found important evidence in his investigations.
Trump has consistently blasted the investigation, stating there is no merit to it, hinting he may attempt to obstruct justice (a criminal offense) by shutting the whole thing down. Shades of Richard Nixon.
While the publicly known evidence is currently largely circumstantial, it is clear Mueller has found something significant, otherwise he would not have impaneled a grand jury – possibly against the president of the United States and his dealings with the Russians.
What do grand juries do?
A grand jury is a legal body empowered to conduct official proceedings and investigate potential criminal conduct, and determine whether criminal charges should be brought. A grand jury may compel the production of documents and compel sworn testimony of witnesses to appear before it. A grand jury is separate from the courts, which do not preside over its functioning.
The United States and Liberia are the only countries that retain grand juries, though other common law jurisdictions formerly employed them, and most others now employ some other form of preliminary hearing. Grand juries perform both accusatory and investigatory functions. The investigatory functions of grand juries include obtaining and reviewing documents and other evidence, and hearing sworn testimonies of witnesses who appear before it; the accusatory function determines whether there is probable cause to believe that one or more persons committed a certain offence within the venue of a district court.
A grand jury in the United States is usually composed of 16 to 23 citizens, though in Virginia it is composed of fewer numbers for regular or special grand juries. In Ireland, they also functioned as local government authorities. In Japan, the Law of July 12, 1948 created the Kensatsu Shinsakai (Prosecutorial Review Commission or PRC system), inspired by the American system.
Further, here’s the purpose of a grand jury in this case, also from Wikipedia:
The function of a grand jury is to accuse persons who may be guilty of an offense, but the institution is also a shield against unfounded and oppressive prosecution. It is a means for lay citizens, representative of the community, to participate in the administration of justice. It can also make presentments on crime and maladministration in its area. The traditional number of the grand jury is 23.
The mode of accusation is by a written statement in solemn form (indictment) describing the offense with proper accompaniments of time and circumstances, and certainty of act and person or by a mode less formal, which is usually the spontaneous act of the grand jury, called presentment.  No indictment or presentment can be made except by concurrence of at least twelve of the jurors. The grand jury may accuse upon their own knowledge, but it is generally done upon the testimony of witnesses under oath and other evidence heard before them. The proceedings of grand jury are, in the first instance, at the instigation of the government or other prosecutor, and ex parte and in secret deliberation. The accused has no knowledge nor right to interfere with their proceedings. 
If they find the accusation true, which is usually drawn up in form by the prosecutor or an officer of the court, they write upon the indictment the words “a true bill” which is signed by the foreman of the grand jury and presented to the court publicly in the presence of all the jurors. If the indictment is not proven to the satisfaction of the grand jury, the word “ignoramus” or “not a true bill” is written upon it by the grand jury, or by their foreman and then said to be ignored, and the accusation is dismissed as unfounded. If the grand jury returns an indictment as a true bill (“billa vera”), the indictment is said to be founded and party stand indicted, and required to be put upon trial. 
President Trump has continued to try to discredit the investigation, both on Twitter and in interviews, but the fact remains, where there’s smoke, there’s fire.
The major obstacle at this point is a Republican Senate and House of Representatives, many of whom who will be reluctant to punish a Republican president, despite his growing unpopularity among his base and his own party as the firings and political failings pile on.
At the moment, the president is on vacation in New Jersey at the Bedminster Country Club doing precisely what he criticized his predecessor for – “excessive” golfing.
There’s no shortage of entertainment value coming out of the White House and elsewhere these days, if you can detach from a particular outcome, as the late, great George Carlin famously did in his later years.