When you play the quest you will get quests in which Zoe and the gang will ask you to send Hearts to friends. Sending hearts means that you will clean up there garden.
Every day you can clean-up the garden of five friends. There will be 5 objects in their garden and if you clean them all up your friend will get a Heart. Cleaning means you tab on the objects. You cannot check if your friend got the Heart. if you play among friends you will see that sometimes it takes a few hours for them to get the gift, sometimes a few days and sometimes your gifts get lost in space.
Not only your friend will get a Heart, so will you, one for every object. Sending Hearts is the easiest way to earn Hearts, 25 a day. If you want to buy your Lion his favorite bracelet you'll need to do this every day for a long, long time. The bracelet costs a 1000 Hearts.