Something that's alarming is disturbing or upsetting. It can be alarming when you hear a sudden, loud crash of thunder.
The idea of climate change is very alarming to many people, and the sound of a police car's siren is alarming when you're driving. Things that are alarming grab your attention, and either frighten you or at least unsettle you. The adjective alarming comes from alarm, via the Old French alarme, which is rooted in the Italian interjection all'arme, a rally or warning cry, "to arms!"