1. The time when sound playback starts on the device (smartphone, etc.)
2. The time when the sound actually comes out of the earbuds.
The time difference between 1. and 2. is the delay of the earbuds.
YouTube or video player apps show the video as late as the delay time of the audio in order to eliminate this delay. So users feel there is no delay. However, since this app does not perform such manipulations, you can experience the delay of your earbuds devices.
In general, devices connected wirelessly, such as Bluetooth, have a longer delay than devices connected by wire.
[How to test the delay]
When the clock hand pass 0ms(milliseconds), the device starts playing 'tick' sound. The delay is where the clock hand is located when the earbuds actually make a 'tick' sound.