Bonus rounds are the rounds that have more specific rules and you get them on round 3.
A bonus round starts with a roll of dice, where you get a random letter. This letter indicates the initial letter of all your answers during this round. As you can see in the example below, the letter C is chosen.
After you tap on the 'OK' button that pops up within a couple of seconds, the bonus round starts. As you can see in the image below, the initial letter is already there, so you don't have to type it again (i.e. for the answer 'Chocolate', you only need to type 'hocolate', as C is already there).