嘀嘀 is an onomatopoeia for short, repeated beeping sounds — most commonly the sound of a car horn (like 'beep beep') or an electronic notification. It's also famously the name of the ride-hailing app Didi Chuxing, but as a sound word it's reduplicated and stands alone in a sentence. Note: the pinyin in the source data is listed as just 'dī', but the headword has two characters; in natural speech it would be pronounced 'dī dī'.