For slides that ask "How do you say...?", the timer is meant to give you time to think of, then say your answer. Whether you say it aloud, or just in your head is up to you as Mango does not record or check your answer. Simply self-evaluate whether you answered correctly or not.
If you need more time to answer, click the "+" icon.
You can also turn the timer off by toggling the switch on the left. Click the orange eye icon or tap the spacebar on your keyboard to reveal the answer when you're ready.