Infill refers to the internal structure or pattern inside a 3D-printed object. Rather than printing a solid mass, most 3D printers create a lattice or grid within the object, which helps balance strength, weight, and material usage. The density and pattern of the infill can be adjusted to make the object stronger, lighter, or more transparent, depending on its intended use. Infill is crucial for optimizing print time and material costs while ensuring the final product meets desired mechanical properties.