What it says on the tin. From what I understand, the Democrat heavily courted Hoover (who, at the time, was an international hero for his relief efforts in post-WW1 Europe) to be their candidate for the Presidency in 1920, so much so that Wilson apparently straight up said that Hoover was his preferred successor. Ultimately, Hoover rejected their offers, partly because he (correctly) guessed that 1920 was a Republican year, and partly because (at least, according to legend) the only Democrat he had known growing up had been the town drunk.
But, this being AH, let's say "What if?". What if Hoover decided to accept their offer and run as a Democrat against, presumably, Harding (or whoever ends up winning the nomination)? Would he have won the election, and if so, what would a Hoover presidency in 1920 look like?