The speech recognition measures how well your pronunciation matches that of the audio track you are hearing.
If it's a good match, a green check mark will appear in the microphone icon and the system will move on to the next vocab item. If it's not a good match, the microphone icon will shake. You'll have the chance to try five times before the system moves on to the next item.
In the apps
Tap the speaker icon to hear the audio track then speak when the microphone icon appears. If your pronunciation is a good match, the vocab item will turn green and the app will move on to the next item. If it's not a good match, the vocab item will shake and turn red. You'll have to try at least one more time before seeing the option to skip to the next item – an arrow at bottom right.