This is your Sentinel’s ship energy. This is used to fire your ship weapons from orbit, and it charges up over time. These weapons can be equipped in the armoury (the weapons available are displayed on the top right, and must be equipped on the bottom row of weapons.
To fire them in the game, press the weapon selector in the bottom right until your equipped ship weapons are displayed and then just press one to activate it.
Glad you’re enjoying the game!
.