The Head Light is, as it suggests, a light on your suit's helmet.
The headlight has 4 modes: off and 3 different brightnesses. The brighter the light the more of your power will be consumed per second.
The headlight can be upgraded later in the game to a Suit Headlight MK1 allowing much more efficient use of power.
The headlight can be toggled with the "L" button and it will cycle through the four available modes in sequence.