FDR was telling Americans that in order for him to get the country out of its economic crisis, he would have to exercise EXPANDED presidential power. In other words, he would have to be able to take decisive action, without interference from Congress, in a way that presidents had never done before. The normal balance of power among the branches of government would be out of whack for a while.
Early on, Congress largely went along with this. Eventually, though, FDR began to encounter more and more resistance from Congress and especially from the Supreme Court, which ruled that some of his programs were unconstitutional.