A working definition
A drug is any chemical substance that, when taken into or applied to the body, produces a measurable change in how the body works. That is deliberately broad. It covers the tablet you take for a headache, the caffeine in coffee, the local anaesthetic at the dentist, and the insulin a person with diabetes injects. The common thread is not where the substance comes from or whether a doctor prescribed it, but that it changes a biological process in a way we can observe.
Drugs work by binding to something
A drug rarely acts by some vague 'strengthening' or 'cleansing'. It acts because its molecules physically attach to a specific structure in the body — a drug target such as a receptor, an enzyme, or a transporter. When a drug molecule fits into its target the way a key fits a lock, we call the drug a ligand for that target. This single idea — molecules binding to molecules — is the foundation of almost everything in this whole ladder.
Often a drug works by imitating or blocking a molecule the body already makes — an endogenous ligand like adrenaline or a natural pain signal. The drug slots into the same target the body's own chemical uses. Understanding what the drug binds to, and what happens next, is what we call its mechanism of action.