“Hallo!” obviously “Hello!” in English. A greeting, a friendly interaction.
Nope, not so obvious. Not here in Germany.
“Hallo!” is actually a way to point out the misdoings of others, to shame them into behaving properly: Get out of the bike lane, don’t cross on the red man, why haven’t you got your lights on, etc.
“Hallo!” must be said in a high-pitched sing-songy, sarcastic and patronising manner for the full affect to be achieved. Germans and especially the older generation of Germans are very practiced at this.
Luckily, it’s not something foreigners have caught on to, maybe because we can’t get the tone-of-voice accurate enough, or simply because this kind of interruption or rebuff on our actions actually bugs the hell out of us!