It's actually the infinity symbol ♾️ It means you've received unlimited lives for a certain period of time. Once you activate it, there should be a little countdown clock accompanying the heart where it tells you how much longer you'll have unlimited lives for.
*May be worth noting: unlike some other games I've played before, the unlimited lives session does not reset your actual lives back to five when the unlimited period ends... so you come back to however many lives you had before activating.