A resource, specifically the term relating to the Devworld, is a material that can be used to create other things.
Resources are generally divided into several classes, organized by the degree of difficulty in obtaining each one. Each resource type also has a visual representation in the user interface, which is provided in parentheses after the name.
Generic (white sphere) - A faintly luminous material that forms the majority of all basic and intermediate products used in the Devworld. Specifically useful for building structures due to its ubiquity and low gathering requirements.
More types as they're made available.
Most basic resources are acquired by either hunting Chaos Constructs or extraction.
Hunting is fairly self-explanatory; the specific resources a Chaos Construct leaves behind depend on the construct's general region, power level, and divergence at the time of destruction.
Extraction is separated into two broad categories, harvesting and mining; the difference between the two is the location at which the extraction is performed. Harvesting is generally understood to mean extraction at the surface of an area, where it meets the sky; mining is carried out beneath the surface, whether literally vertically beneath it or while tunneling into the side of a structure.