Trump is still a subject of the investigation, which means he's a person of interest and his actions are within the scope of what Mueller is investigating, and his conduct could potentially be criminal. Whereas the target of an investigation is someone who the prosecutor believes has committed a crime, is likely to be charged, and there is substantial evidence to prove guilt.
The fact that that Trump isn't a target at the moment likely means that Mueller doesn't think he has enough evidence against Trump, now, to convict him of a crime (i.e. there's currently no "slam dunk" case).
Another possibility is that Trump isn't a "target" simply because Mueller has no intention of charging Trump with a crime because he's concluded he legally can't do so (it's generally agreed, and is the position of DOJ, that you can't indict a sitting President).
On the other hand, for a variety of reasons I don't find persuasive the argument advanced in some quarters that Mueller isn't calling Trump a target simply to keep the President from firing him or trying to destroy evidence, or is attempting to "lure Trump into a false sense of security" in an effort to get him to speak under oath (the latter of which would possibly be unethical).
Again, Trump could certainly go from being a "subject" to a "target" at any time (these designations can be, ah, squishy), though for the moment this news would seem to undermine the breathless, and endless talk of smoking guns and open and shut cases. In any event, Trump isn't off the hook, and if I were his lawyer I'd still advise him against talking to the Special Counsel.
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