It’s basically a scheme for handing over public assets to private corporations that will extract maximum profits via user fees and tolls.
Here are more of the details from an article at Slate:
Under Trump’s plan…the federal government would offer tax credits to private investors interested in funding large infrastructure projects, who would put down some of their own money up front, then borrow the rest on the private bond markets. They would eventually earn their profits on the back end from usage fees, such as highway and bridge tolls (if they built a highway or bridge) or higher water rates (if they fixed up some water mains). So instead of paying for their new roads at tax time, Americans would pay for them during their daily commute. And of course, all these private developers would earn a nice return at the end of the day.“Trump’s plan” is “massive corporate welfare plan for contractors” and the “tax breaks”…”could all go just to fatten the pockets of investors in previously planned projects.”