Passive periphrastic is a fancypants English-language Latin grammar term for "when you use the passive participle thingy plus the copula to indicate something 'must' or 'needs' to happen." So "Carthago delenda est" is pretty much the most famous example of the passive periphrastic--"Carthage is to be destroyed" = "Carthage must be destroyed." It's like one of the few things I remember from Latin when I took it 15 years ago.