Okay. The top bar is called

Crystal Energy

You can purchase the Crystal Energy by clicking on the top bar, and there will be 3 tabs.
1. Purchase with money
2. Pay higher rates for instant energy from Crowns => CE, or CE => Crowns
3. Market tab, LEarn how to use it and you can find ce for cheaper prices :)

This is the energy you cannot get for free and will have to either trade currency (Using market)
and change Crowns => Crystal Energy, or Vice Versa. Crystal Energy is used for Crafting Higher
Tier gear as your normal mist cannot pass 100 and some crafting recipes require 200,400,800.
You can also do other things with over 100 energy, like buying Trinket slots, or Silvers keys.

The bottom bar is called

Mist Energy

This is the Energy that regenerates over time, it is Approximately 22hours to accumulate 0 energy to 100 energy.
You can use this to buy things and you can also "share" mist with ce crafting, for example the recipe requires 400 energy,
You can use 100 Mist Energy + 300 Crystal Energy, or you can buy a trinket slot with 100Mist + 50 Crystal Energy.
And you can use mist for clockworks, 10 Energy per Elevator.

IF this is bit hard to understand try Spiral Knights Forums