Security rings are giant, round gates of Luminoth design. Two of them act as safeguards near the entrance to the Sanctuary Fortress, in the Temple Transport Access room. The rings block archways in Samus' path. To lower them, Samus has to use Spinner devices and charge them until the rings are fully retracted. They will remain this way for the rest of Metroid Prime 2: Echoes.
"Object scan complete. Security ring is locked down and active. You may be able to access its control system and move it to open position."