Growth: Makes the plant mature faster. Over 23 it will make the plant act like a weed, while being immune to weed itself.
Gain: Makes the plant yield more on harvest. There is no special condition for this one. The higher, the better.
Resistance: Makes the plant be more resistant to weeds and trampling. Also gives you a better chance of recovering a seed bag when left clicking a mature plant. Over ~20 (I dun remember the value) it will make the plant take longer to breed.
All stats maxes on 31. Crops with higher stats (and tiers) require better environment conditions to grow, otherwise they may grow slower, not grow at all, or simply die (the crop will vanish).
Environment conditions are soil, biome, height, space, sunlight, fertilizer and hydration.
Better environment conditions also makes crops grow faster.