Nuggets are a type of material added by vanilla Minecraft and Thermal Foundation. They are the smallest units of metal; one Nugget of metal is worth a ninth of an Ingot of the same metal. Vanilla Minecraft and Thermal Foundation add Nugget types for all of their metals and alloys.
Nuggets are somewhat commonly used in crafting; mostly when a metal is needed for a recipe, but using Ingots would make that recipe too expensive.