The Materials storage is a camp building that is used to store more material resources. With each additional materials storage you built, 6 extra storage slot are granted. Please note that all materials storage share the same value of materials stored, therefore if you have two materials storage with a specific items at 10, then you only have 10 of this item, not 20.
To learn the blueprint for this building, you must find 4 incomplete versions of it in a specific zone. You can tell how many pieces you've found in a zone by clicking the map icon at the top left.