So how exactly do you agree adjectives with plural nouns? There seems to be no change here, but is it different for different cases?
To is a demonstrative pronoun—this/that. "Są" is the plural form of być, or "are."
"Jabłko to smaczne" is a grammatical sentence, but it's more like: "An apple—that is tasty" (or perhaps more colloquially: "Apples—those are tasty."). This is simply "apples are tasty." The singular would be: Jabłko jest smaczne.