What does the red bubble with a number in it mean in prize draw?

Olivier de Lamotte -

When you are selecting winners, you may get a red bubble with a number in it.

This just means that this participant has already won a prize from a former campaign. It is up to you to decide if this person can win the prize for which he has been drawn, according to your rules.

You can double check the results in the CRM section, sub-section 'Participants.