The number shown on your App indicates the number of times your phone exchanged Bluetooth signals with other phones that have TraceTogether activated. The longer the phones are in one another's Bluetooth range, the more signals they exchange. The counter resets everyday.
Here's what happens:
1. The App logs all exchanges with nearby TraceTogether phones. Because Bluetooth's range is about 10 metres (and could be more), phones close to you such as your neighbours' could exchange signals with your phone, sometimes through the walls/ceiling/floor. But not to worry, because...
2. When a patient confirmed with COVID-19 uploads their Bluetooth proximity data to MOH, it is processed and filtered based on duration and signal strength to identify only the close contacts.
Filtering is not done on the phone because it's not a straightforward cut-off. The algorithm calibrates the data to account for different transmission powers of different phone models.