「」。
What it means is that debris flows happen after a mountain fire.
Literal
That [topic-は] mountain-fire [gen-の] after [at-に] 'debris-flow' [subj-が] occur [quot-という] thing [copula-です].
A definition-flavored sentence with ~ということです as the softened explanatory ending, literally 'it's the matter of saying that X' = 'what it means is X / the upshot is X.' 土石流 'debris flow' is a compound 土 'earth' + 石 'stone' + 流 'flow,' standard disaster-terminology vocabulary. The 「 」 quotation marks around 土石流 highlight it as a defined technical term.