From Halopedia, the Halo wiki
The Goliath is a superheavy infantry unit deployed by Colony to protect areas it finds of particular importance. It is walking tank that bulldozes foes with its powerful melee smash attacks, crushing anything that gets in its way. What it can’t immediately destroy with its smashing blows can be dealt with using the Goliath’s ram ability, which does massive damage and stuns the target temporarily. The Goliath's Rage upgrade significantly increases melee and ram damage, making it even more of a terror against enemy vehicles.
Unlike a typical Mgalekgolo pair, which splits a colony into two independent bodies, the Goliath is a Hunter-style body containing a single colony that optimizes brute strength over all other considerations. Inside the Goliath’s armored shell most of the Lekgolo eel-worms in the gestalt adapt to function as corded pseudo-muscle and thick-skinned protective bands, with a minimum of sensory and cognitive function. Goliaths are not easily self-sustained due to their ravenous hunger for nutrients, and are typically only seen in times of extreme danger among feral Lekgolo, but Colony can coerce or direct subsidiary Hunter gestalts into this form seemingly at will.