I might be able to help. The Church's traditional definition of a martyr is somebody who's killed specifically because of their Christian faith. "In odium fidei" basically means "for hatred of the faith" -- in other words, people who are killed because their murderers despise Christianity.

Some have argued that people like Oscar Romero, Stan Rother and the women killed in El Salvador don't fit that definition because they were killed (purportedly) for stirring up social unrest, not because of their faith. What Pope Francis is saying is that these people are indeed genuine martyrs because it was their faith that compelled them to serve the poor in the first place, thus putting their lives at risk. 

Francis just wants the Church's official understanding of martyrdom expanded to include situations like these, and his proclamation of Oscar Romero and, now, Stan Rother as martyrs is his way of trying to establish that precedent.