The Grid is an Espheni planetary defense.
The Grid is a series of energy beams created and connected to the Motherships, which act as nodes located in many major cities all over the Earth. Prolonged exposure is said to be capable of irradiating all life on a planet. The Espheni and their forces are immune to this radiation through genetic engineering. When activated it can prevent any hostile ship from entering the atmosphere. The Grid is extremely fragile however, destroying just one of the Motherships can cause the whole grid to deactivate.
Almost a year and a half after the Espheni invaded Earth they activated the Grid in an attempt to prevent the Volm from landing their army to aid the Global Resistance. An advance force of Volm however had anticipated this and developed a cannon capable of destroying a Mothership. They launched an attack on the Mothership in Boston and destroyed it causing the Grid to deactivate, allowing the Volm to land.