The Containment Shield was an energy shield wall within the Sentinel Wall around Installation 05's Library, built by the Forerunners. It helped contain and enclose the Flood inside the Quarantine Zone, and keep out anything that could sustain the Flood. It also appears to be a sort of energy field that doesn't allow anything through either way. It had to be lowered and deactivated in order for Covenant forces to enter the Library and recover the Activation Index.