Extra layer of cyborg protection
In certain places you'll come across a glowing red ball in a transparent cube. This is a Covenant 'overshield' that you can apply by stepping over it. Acting like an extra layer over your normal shield, it soaks up damage before your normal shield starts to be affected. This allows you to take much greater fire before you start to lose health. It also lets you survive huge falls, something which is quite useful with certain bits of fun such as up on top of the island in level 4. Nice of the Covenant to leave them lying around eh?
If you wish to get rid of a damaged overshield so you can apply a new one, there are several ways to do it. Take a bit of enemy fire; fire bullets or needles against a reflecting surface such as a stationary shield, so they come back and damage you; set off a grenade nearby; take a damaging fall; or let yourself get burnt by fire if there's one nearby.