Storage is the space in which an item is contained. A tile in the environment has 4 storage spaces for items.
Items such as components cannot stack and therefore take up 1 storage space, whereas a resource such as metal ore can be stacked to 20 to fill the same space. Storage slots cannot be shared, for example: you cannot have both 10 Metal Ore and 10 Crystal Chunks in one storage slot.
Foundation tiles are an exception: they are created in sets of 5 and can be stacked up to 40 units in one storage slot.