Power is required to fulfill the production in various buildings:
 
 
You can get more power from the Power plant. To get additional power, you will need some resources and coins.
 
 
Initially, you have a certain number of power units available on your profile, but as you level up, the number of available power units increases. To be able to launch more production processes at once, you need to upgrade your power plant to have more power units in store.
 
If you’ve reached the maximum number of power units for your current level, the game will show you when the next upgrade is available:
 
 
Once you reach level 5 and have the workshop completed, you will receive a quest to find the power plant on your farm and restore it:
 
 
NOTE: you can only launch so many production processes simultaneously, as you have power units available at the moment.
For example, if you have 20 power units total, and you have used up 13 of them in several production processes, you will have 7 units left, so you won't be able to start new production requiring 8 power units until at least one of the ongoing processes is completed.
In case there are no spare power units at the moment, you just need to wait a bit until the power units that are currently deployed become available again.