From Brotato Wiki

Materials

In Brotato, Materials are small green objects that are dropped by Enemies when they are killed. Harvesting also grants you materials at the end of each wave.

When you gain materials, you gain both XP, used to level up, and Gold, the currency used to reroll and buy items in the Shop

Within the game, there is no clear distinction between Materials and Gold, and the terms are used interchangeably despite functioning in two different ways. We make the distinction here on this page to make things more clear. The rest of the Wikipedia uses the terms in the same way as the game does.

Gold

Gold is the currency used to reroll Upgrades and items in the Shop. And it is used to buy items in the Shop. In the game, the currency is just referred to as Materials.