An Associated Press article by Chad Day and Eric Tucker published Saturday points out that while Robert Mueller's report has not yet been turned in to the Justice Department, let alone made public, a lot of its likely contents can already be inferred from publicly available court records, such as indictments, plea agreements, sentencing memos, etc.
For example, it's clear that the Russian government had started preparing to interfere in the 2016 election at least by 2014 and in the spring of 2016 had focused on opposing Hillary Clinton. Their efforts ranged from breaking into computers and email accounts associated with the Democratic Party and people affiliated with the Clinton campaign to placing a multitude of pro-Trump and anti-Clinton messages on social media using false identities.
Meanwhile many persons associated with the Trump campaign were in contact with affiliates of Russian intelligence and in many cases were interested in obtaining politically damaging information the Russians had or claimed to have about Clinton. Those involved ranged from a young foreign policy advisor named George Papadopoulos to such senior people as Paul Manafort (who had previously made millions as a political consultant to the pro-Russian side in Ukraine and later for a while headed the Trump campaign) and perhaps most famously Donald Trump Jr.
In fact, when it became clear that the news media were about to release a chain of emails setting up a meeting with Russians claiming to be ready to pass on dirt the Russian government had supposedly obtained about Clinton, Trump Jr himself released that email chain via his Twitter account. He also acknowledged that the meeting in question took place in Trump Tower on June 9 and included, besides the Russians and the younger Trump himself, campaign head Paul Manafort and Trump's son-in-law Jared Kushner. According to those from the Trump campaign, the promised dirt on Clinton was not forthcoming and instead the Russians wanted to talk about the possibility of reducing sanctions imposed by the U.S. under the Magnitsky Act in return for an end to blocking American adoptions of Russian infants.
(More recently the Trump administration as been criticized for failing to enforce the Magnitsky Act; see this post from a few days ago.)
During the investigation that continued after President Trump took office, various members of the administration lied to the FBI about their direct or indirect contacts with Russia, including national security advisor Michael Flynn, campaign advisor George Papadopoulos, Trump lawyer Michael Cohen, et al, who ultimately entered guilty pleas.
So far nothing in the public record related to the investigation appears to directly implicate Trump except for falsely denying the obvious collusion between Russia and the campaign took place. In addition, a number of the people having known or alleged contacts with Russian intelligence have not been indicted for, much less been found guilty of, any crimes. It's likely that there's a good deal more information to be included in Robert Mueller's report, should it ever be made public.