Raiding is an action by which you send Units to enemy Bases in order to take Resources by force. The only Resources that can be raided are Fuel, Munitions, and Rations.
You will need to make sure that the Squad you send on a Raid is powerful enough to overcome the forces stationed at the target Base, and that they have enough Carrying Capacity to transport any seized Resources back to your Base. Both the offensive power of your Squad and its Carrying Capacity can be seen when you are selecting Units to send on the Raid.