From Halopedia, the Halo wiki
Smartmatter is a type of material often used in Forerunner construction. It is composed of self-assembling nanomachine blocks reinforced by energy bonds. This nature allowed smartmatter to be "grown" in the Forerunner assembler vats in a manner similar to biological life - a process that would later be mimicked by the Covenant and their use of nanolaminate in Assembly Forge manufacturing. Growing creations in this manner allowed smart matter to assemble structures and objects with atomic-level precision, and allowed the Forerunners mass production of exotic materials that cannot exist in nature - including hyperdense masses.
Smartmatter-assembled objects which take too much damage run the risk of causing the bonds in the material to fail, and the machine cells to lose cohesion, which results in the entire assembly eventually disintegrating into fine dust and hard light embers. Smartmatter was able to help Forerunner-built structures last eons with minimal maintenance.