Here you can practice typing any (but only) letters that you want, tap the button for a dit and hold the button (just under half a second) for a dah.
The bar under the text field is a countdown to how much time you have to finish the letter, every dit and dah you type resets the countdown, you have to be fast and consistent. Once the countodwn reaches the end, it means that you have finished your letter.
If the typing button turns red, it means that you made a mistake and tried typing a letter that doesn't exist.