To be specific, you can now pay 5 Farm Cash to send a single friend a free Javelin Cow, as a way of saying thanks for all of their help. If you send one or more cows, you'll receive a free FarmVille Games Tree for your own game. If you're familiar with these events, you'll know that it's rather easy to partner up with a friend to receive both items, as you'll simply need to send them a cow, and have them return the favor. In that way, you'll both spend just 5 Farm Cash, but will wind up with both the Cow and a FarmVille Games tree for your farms!
It should be noted that if you don't see a particular friend that you wish to thank, just close this in-game window and relaunch it. That should give you a new selection of friends to choose from. This feature will likely only be available for a limited time, so make sure to send out some cows as quickly as you can, if you want to participate at all.