Biden is currently winning yes, but the remaining states that have not been called yet currently are leaning Trump, and those going to trump would launch him past biden to victory.
The trouble is that Trump wants those votes to stop being counted now while he's ahead, when there is plenty of room for Biden to pull ahead in those states. Trump has run 80% of the race they passed the last checkpoint with biden in the lead so on all the scoreboards biden is winning, but Trump is physically ahead of biden in the race for what is essentially the first time since the race started but he's getting winded just as biden seems to be getting his second wind. Trump sees this and is trying to tell the refs that he(Trump) is ahead and has clearly won and wants them to call the race early.
The question is no president ever has tried to do that before, consensus seems to be "You can't do that" But that has never stopped Trump from doing something he can't do before, because no-one seems to be intrested in reining him in when he oversteps his station, and now the part of the government that is supposed to say what hee can and can't do, is basically in his pocket.
EDIT: Oh and ignore anything you see about a popular vote, that means nothing in the US. As much as we claim to be a direct democracy we are not. Every state gets a number of votes based on the state's population +1 (or +2 I forget) The way everything works, each state votes for president, whoever wins in that, with a few exceptions, gets the WHOLE STATE. I am in Florida. I voted Biden. But because Trump won in Florida I have in effect voted Trump. I physically voted Biden, but because Biden didn't win my state my entire state voted Trump, no matter what I have to say about it.