When we visit our friends farms, we can use the ‘black glove’ icon to steal crops that are ready to be harvested.
We can only steal a certain amount of times per day from each friend, the number next to this shows how many we have left to steal (achieving different trophy levels for ‘crops and robbers’ affects this amount_

Each time we steal once successfully – we steal 2 crops from each farmer.
Each plot of crops can only be stolen from a maximum of 8 times, after this the farmer gets to keep whatever is left (so farmers are guaranteed the ‘expected crops number’ minus 16)/
1 Green Point is gained from stealing crops, but it takes 2 notches from our stamina bars.
Once we have stolen from somebody the maximum number of times for that day, we have to wait for the game reset at 12am GMT to be able to steal again.

Stealing can benefit us if we need a few extra coins to be able to afford something, if we need a few extra crops to complete quest or to gain a couple of extra green points to level up.