Sedevacantism is the belief that the man calling himself Pope Francis is not the real Pope, and none of the occupants of the Holy See since 1958 have been real Popes. They believe that the decisions of the Second Vatican Council were heretical, and those who endorsed and propagated them are therefore heretics, even or especially if they are called themselves 'Pope' at the time. Not all of those who believe this draw the same conclusions from it, so it is easy to overgeneralize about Sedevacantists.
Not all those who strongly disagree with Vatican II are sedevacantists, only the minority who believe both that those Popes who endorsed it made themselves heretics, and that by doing so they automatically made themselves ineligible to be Pope. Arguments have been made for and against these positions using Church law.
Groups of Sedevantists have sometimes gone so far as to elect a 'real' Pope, although there are no known cases of attempted military action against the alleged Pope.