Materials is a currency in Game Boy Wars 3. It is used along with Gold to build units.
Materials can be obtained by controlling Factories. Every 20 strength a Factory has provides 1 unit of Materials--in other words, the standard is 5 units.
As mentioned above, Materials is needed to build units, as well as repair them. Also, when the Constructor is resupplied at a Factory or HQ, it requires 1 unit of Materials to restore each unit of its Materials that it is missing in addition to the standard Gold cost.