Guardians were huge Dreadnought-sized Sentinels built by the Forerunners to police backwater planets in their Empire, which were deemed useless against the Flood by the idiot Forerunners despite their awesome power and stored away on random planets for any old idiot to find.
One Guardian was placed on Sanghelios, and formed the foundation of the Elites religion before the founding of the Covenant.
One Guardian was placed on Meridian, which was reactivated by Cortana and used to slaughter the population after her defection to the Help the Prometheans charity.
A Guardian fully deployed
The face of a Guardian