A drug is a molecule with a job
A drug is, at heart, a small piece of matter — a molecule — that you put into the body to make something specific happen: lower a fever, stop a clot, calm an overactive nerve. It is not magic and it is not a vague 'substance'. It is a particular shape, made of particular atoms, that does a particular thing in a particular place. Most of the pills in a medicine cabinet are a kind of drug called a small molecule: compact, often swallowable, small enough to slip into the tight crevices of the body's machinery.
The reason a drug can be so selective is that the body is full of distinct molecular machines, and a well-made drug fits one of them the way a key fits one lock. That fitting and the change it causes is what we call the drug's bioactivity — its measurable effect on a living system. A substance with no bioactivity is, by definition, not a drug; it is just a chemical.
Where the drug goes: the target
The 'one lock' the drug fits is its drug target — usually a protein in your cells whose job, when nudged, changes how you feel or heal. A very common kind of target is a receptor: a protein that sits like an antenna and listens for the body's own signals. A drug can either mimic those signals or block them. The act of the drug actually touching and altering its target is the mechanism of action — the honest, physical answer to 'how does this medicine work?'
Finally, every drug is approved for a reason — the disease or condition it is meant to treat. That is its indication. The same molecule can have more than one indication, and a molecule with no clear indication is a science project, not yet a medicine.