Residue is anything that's left over when a substance has been removed, like the grease left over on a frying pan. It can also mean, simply, "remainder."
When residue refers to a liquid, it’s what’s left at the bottom of a bottle, a pot, or a can after the rest has been poured out. Legally, the residue is the remainder of the money in an estate, after bills and taxes have been paid. So if your millionaire aunt leaves you her estate, but didn’t pay any bills for the last ten years, you may not get much residue!