That’s the PRCD - Portable Residual Current Device, which is to protect you from a shock if the charger malfunctions due to any manufacturing defects.
The red button is the Reset button. It is to be used if the charger trips.
The yellow button is the Test button, to check if the PRCD is working.
It is recommended to check the PRCD once a month. To check, you’ll need to press the test button. If the power cuts off to the Dot, it’s working correctly. Press the reset button again to restore the Dot after testing.
This info is also available in the user manual for the Dot.