Hydrocarbon extraction uses light hydrocarbon solvents, mainly butane and propane, to pull cannabinoids and terpenes out of cannabis. Products made this way are often called BHO (butane hash oil) or PHO (propane hash oil). It is the method behind most shatter, badder, sugar, live resin and diamonds and sauce.
In a licensed lab, this is done in a closed-loop system. Chilled solvent is washed through the plant material, dissolving the trichome contents. The solvent-rich mixture is then collected, and the solvent is recovered and removed, usually with gentle heat and a vacuum oven. That removal step is called purging.
Hydrocarbons are popular because they are very good at capturing terpenes at low temperatures, which helps with flavor, and because extractors can steer the final texture by how they purge and handle the extract.
Butane and propane are flammable, which is why extraction belongs only in professional, permitted facilities. Never attempt it at home.
A properly made hydrocarbon extract should have only trace residual solvent, within regulatory limits. That is what the residual solvents section of a COA checks. If you are curious about a product, ask to see its lab report.