With Crop Mastery, you will notice that crops has 3 stars which is the crop level. For one star, you need to complete the mastery number of each crops. One mastery plot is equivalent to 1 mastery. While mastering a crop you can earn some special rewards. Master each of a crop’s 3 levels for a special bonus
Here's the official statement:
"Congratulations! We're happy to introduce Crop Mastery! Now
harvesting a crop will earn you a point towards that crop's mastery level.
Master each of a crop's 3 levels for a special bonus!"