Mastery refers to how long it takes to plant or harvest 1 plot of land.
With each star you earn you knock off 0.1 seconds/1 plot — so you can go from 1 second to plant or harvest 1 plot to just 0.3 seconds/1 plot if you get all the stars.
ATTENTION: This does not change or reduce the growth time of Crops! — only how fast you can plant and harvest your crops.
If you have a small farm it does not matter so much, but if you have a huge one, with over 1000 plots, it adds up.
There is an indicator in the Mastery Tab telling you exactly how much time you save with Mastery for a particular crop when planting and harvesting.
The RC number under the stars isn’t a prize you can earn, but a button indicating what you have to pay to get the next star instead of harvesting X crops when you press it.
The blue text says “Harvest X more Y plots for next star or get it now for Z RC.