There are 60 Missions in this game, so this achievement is for getting the Golden Fleece on 1/3rd of them.
A Golden Fleece is a hidden little flocker somewhere in the map that you must get your sheep to reach them so they can unlock them from their cage. The picture below is to the Golden Fleece on the 1st Mission "School of Flock".
The locations of the Golden Fleeces varies GREATLY, I suggest getting through all the levels first and seeing how many you notice and can collect. Then just start at the beginning at use the videos to knock more and more off the checklist.
https://www.youtube.com/playlist?list=PL3vg8dgpQ27Xl9QhXmJu5...