What is the heart symbol above a citizen's head?

After your city has aquired a certain number and level of buildings, you will start to see hearts appear above citizens' heads. This just means they now have their dream job!

Assign the citizen to work at the appropriate building to begin their dream job and you'll see "happiness" (associated with the heart symbols) appear at City Hall. This is an important indicator of your city's economic growth, in addition to prosperity level and coin production.

PS: Currently happiness is an independent indicator and will not affect other values.