Other than Hillary Clinton and Donald Trump themselves, few individuals had a greater ability to affect the outcome of this election than James Comey. His late-stage announcement Sunday that Clinton would (again) not be charged with any criminal activity relating to her private email server will certainly affect the election, though perhaps less than his earlier announcement, criticized for its vagueness, that additional emails were being reviewed at all. The reasons the FBI isn't charging Hillary Clinton with criminal wrongdoing are much the same as before, according to Comey's most recent statement:
... We reviewed all of the communications that were to or from Hillary Clinton while she was Secretary of State. Based on our review, we have not changed our conclusions that we expressed in July with respect to Secretary Clinton.
In July, Comey and FBI investigators indicated that they would not charge Secretary Clinton with any criminal activity for her use of a private email server. A criminal charge, Comey said at the time, could only arise from deliberate mishandling of classified materials, and the FBI did not find that Clinton intentionally sent or received anything classified. No reasonable prosecutor, he said at the time, would bring a case against Clinton.
According to Kurt Eichenwald at Newsweek, it was clear to FBI investigators from Comey's initial announcement that most of the "new" emails in question were duplicates or were personal; the FBI's additional review, then, may have only been as simple as confirming that the emails in question had already been reviewed by investigators earlier this year.
Shortly after the news broke, a right-wing conspiracy theory surfaced on Twitter; the theory, promoted by Trump himself, posits that to review thousands of emails in nine days would be impossible. However, many technology experts, including WIRED Magazine and Edward Snowden, came forward to explain that filtering technology would certainly enable that timetable.
Regardless, the key thing to understand about the November and July investigations is that criminal activity would only have occurred if investigators found that Clinton somehow intended to transfer or jeopardize classified materials, which they didn't. The calls of "lock her up!" that have become so ubiquitous on the right are more about wishful thinking on the part of conservatives who want Clinton gone than concerned citizens who have studied the relevant law. If they do want to defeat Clinton, they'll have to do it the old-fashioned way: at the ballot box.