it means one of your neighbors visited your town and polished it
(polishing is done to happiness providing buildings and doubles there provided happiness for 24 hours)
hover your mouse over a flower garden and then the flower garden with the star to see the differance for yourself
so be a nice neighbor and return the favor
, and before you ask what motivating those(you'll see when you visit your neighbor). it will double the next outcome of a coin/production building.