Anti-Pope means a false Pope
in more chaotic times of European History there were times there might be complications that would end with several claimants to the Papacy
I would not say that "many" Catholics think Pope Francis is a false Pope
there is an extremely small minority of Catholics who claim there has been no valid Pope since 1958, this view is called "sedevacantism"
Eric Gajewski has a slightly more nuanced view, he has serious disagreements with Popes from 1958 onward, but he recognizes them as Popes, all except Pope Francis, I forget his exact justifications now
Historically, not all Anti-Popes were bad people, some were elected by a group of Bishops after a new Pope was already elected, but the two groups of Bishops might not have been in communication because of war at that time, but any false Pope would be called an Anti-Pope
St. Robert Bellarmine in his book De Romano Pontifice talks about 5 views on if a heretic can be Pope
now St. Robert Bellarmine is a doctor of the Church, that means his teachings are held up as a definitive way to teach the faith, but also the way he presented the issue shows that this is not an open and shut case