Benjamin Wittes writes at the highly-regarded Lawfare:
The president should wait before popping the champagne corks over this and tweeting in triumph. Yes, in the best-case scenario for the president, Mueller is not proceeding further because he lacks the evidence to do so. But even this possibility contains multitudes: everything from what the president calls “NO COLLUSION!” to evidence that falls just short of adequate to prove criminal conduct to a reasonable jury beyond a reasonable doubt—evidence that could still prove devastating if the conduct at issue becomes public.
There are other possibilities as well. It’s possible, for example, that Mueller is not proceeding against certain defendants other than the president because he has referred them to other prosecutorial offices; some of these referrals are already public, and it’s reasonable to expect there may be other referrals too.
In this iteration, what is ending here is not the investigation, merely the portion of the investigation Mueller chose to retain for himself. It’s possible also that Mueller is finished because he has determined that while the evidence would support a prosecution of the president, he is bound by the Justice Department’s long-standing position that the president is not amenable to criminal process.
You should read his full analysis.