What is the Get Energy screen?
This screen provides a couple of options for you to regain Energy.
You can either spend 100 Gold to completely refill your Energy bar, or watch an advert to get 4 Energy (or 2 Energy in PvP and Events) for free.
Note that you will also be taken to the Get Energy screen when you try to start a Mission when you don't have enough Energy.
Also, note that Campaign, PvP and Event modes each have their own kind of energy and are not interchangeable.