The Grid was the name given to Lorien's vast defense and monitoring system put in place by the Lorien Defense Council.
It is a system that consists of sophisticated sensors that scans and registers the goings on inside the city including the flow of people and goods, the information is compiled for The Munis to keep everything running perfectly. It's lesser known function is the advanced defense system which monitors the planets airways for potential intruders. It controls a string of protective shields and counter attacks situated above the skyline.
While students at the Lorien Defense Academy, Rapp and Sandor are sent to repair the grid systems when their classes are cancelled for the day, two weeks before the attack on the planet. There are multiple locations they are sent to: Sandor goes to work on the grid system outside The Chimæra, where he completes his work a lot slower than Rapp. Sandor notices that damage to the grid, and subsequent repairs, happens far more often than it should; after the Mogadorian attack on Lorien, he realises that the violet beam of light that had appeared was draining its power and making the city more vulnerable, leading to the eventual attack.