There's obviously something in there that he doesn't want anyone to see. 

The most innocent explanation is that they'll show that he's not as wealthy or successful as he claims.  The snippets of his tax returns that we've seen suggest that he's far less wealthy than he claims.  This would be personally embarrassing, and undermine the supposed argument in favor of him being President.  But it wouldn't be a huge problem legally or morally. 

The other option is that there's borderline, or even totally, illegal stuff in there.  We know that Trump has been involved in tax fraud before.  He and his siblings defrauded the New York and US governments of hundreds of millions of dollars in tax schemes related to his father's estate.  It wouldn't be at all surprising if he engaged in tax sheltering strategies which skirted, or even jumped right over, the boundaries of legality.  Now his supporters argue that this can't be the case because any illegality would have been detected in an audit and resulted in criminal charges.  But this represents a lack of understanding of how the IRS operates.  They don't audit everyones taxes.  They mostly take it on faith that you're reporting things right. In fact, due to staffing cutbacks, the IRS has reduced the amount that it audits high earners like Trump.  Now, Trump does claim that he can't release his returns because they are under audit.  First off, that's completely false.  He can release them.  Secondly, he's never provided any proof that his returns are under audit. 

He knows that those who clamor the loudest for their release have no intention of attempting to understand them in fact & kind ( nor can they). And just want factoids to manipulate as a tool against him. He's loud, he's inarticulate, but he's also aware and savvy. 

Obviously, because releasing his tax returns would indeed remove all doubt.  Trump is a crook. 

Only criminals and people that have a lot to hide keep this sort of information from the public.