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.
Materials Limit
There is a limit of 50 materials that can be on the map at once. If more than 50 materials would drop on the ground, the new materials is added to the value of one of the already existing blobs instead of creating a new green blob.
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.