This is your energy bar, which displays your current energy level as a horizontal bar. When your energy level drops below 75%, the bar will turn yellow and it will start flashing. If you drop below 25%, the color will change to red. When the bar turns yellow or red, your remaining energy will also appear next to your ship. This is very important, because you don't have time to look at the bar on the top when in the middle of a battle. It's especially handy when you are at 200 energy in the middle of a base full of shrapnel.