This is a great question, and we'd be happy to help clear things up for you!
Pot Heads are single-use items that are needed in certain quests that require just that little extra Pot Head push ;) "Single-use" means that they don't take up any space on your farm, and aren't something you keep around in the long term. When you receive them, they go directly to your gift box, where you can find them and click "Use" to help you achieve quest goals.
Pot Heads can be earned in several different ways. First and foremost, we'll sometimes give them away on the fan page, so keep a look out there! You can also get them from your news feed through the shares of friends and neigbhors. Finally, many different themed quests will give out different types Pot Head "crowds." These are awesome repeaters that chill on your farm and produce lots of Pot Heads, and send them right into your giftbox once you collect from them. Definitely keep an eye out for Pot Head crowds as rewards during different themes, and be sure to save your Pot Heads up for whenever you might need them!
Hope this helps you out!
Have fun. ;-)