Question: What is the Security System?
The term "Security System" was first introduced in Episode 1x23, Exodus Part 1.
Answer: In Episode 1x23, Exodus Part 1, a group runs away from the monster. Kate asks what it was and Danielle Rousseau
says that it's a security system with the purpose of protecting the island.
In Episode 2x23, Live Together, Die Alone, Desmond remembers Kelvin going below the hatch with a key for the security system. Kelvin explained that the system was a failsafe.
When Locke locks Eko out of the computer room to see what happens when the button is not pressed, Desmond goes below and uses the key. The hatch implodes, but Locke, Desmond, and Eko seem to be okay.
