The Hideout is an intermission location. It is where you will be sent during the "Standby Phase". According to Wataru Frazer, the Hideout was originally a unit for the Sumeragi Family.
The Hideout is where you can increase your bond level with your allies. This will give you more SAS abilities.
(Check Bond Level for information)
A short break where you can restock items, buy weapons and other equipment and increase bond levels.