It's traditional for American presidents to put their financial assets in a blind trust when they become president, meaning that they still own what they own, but for the duration of their time in office they have no control over the assets. This isn't a legal requirement, but it's a customary way for chief executives to set people's minds at risk over the obvious conflicts of interest inherent in actively managing a stock portfolio while also serving as leader of the free world.
Donald Trump has indicated in the past that he wouldn't follow this practice, and the particular nature of his assets — many of which literally have his name on them — would make blindness difficult to achieve. Be that as it may, in a Thursday midday tweet he for the first time said clearly that if he wins the election he intends to seek to personally profit off his office.
After the litigation is disposed of and the case won, I have instructed my execs to open Trump U(?), so much interest in it! I will be pres.— Donald J. Trump (@realDonaldTrump) June 2, 2016
Trump University is pretty clearly a scam, for which Trump is seeking to avoid legal accountability through a racist smear campaign against a federal judge and campaign contributions to state attorneys general. Serving as president would presumably give Trump new tools with which to help get the "university" (which, to be clear, is not in any sense a university) out of legal trouble and create invaluable opportunities to publicize it.
You might wonder why a person who says he's worth $10 billion is so eager to find new ways to make money, and the answer may well have something to do with the considerable evidence that he's massively overstating his net worth.