A drug needs something to grab
A drug does not do anything by floating in your blood. It works only when it physically attaches to a particular molecule in the body and changes what that molecule does. That molecule is the drug target — most often a protein. The study of what a drug does to the body once it binds is called pharmacodynamics, and almost all of it begins with this single act of binding.
The most famous kind of target is the receptor: a protein whose normal job is to receive a chemical signal from the body and pass it on. Any molecule that fits into a receptor and binds it is called a ligand. Your body already makes its own ligands — the endogenous ligands, such as hormones and neurotransmitters. A drug is essentially an outside ligand we add on purpose.
Lock and key, but a soft one
The oldest and still most useful picture is the lock and key: the target is the lock, the drug is the key. The exact pocket on the target where binding happens is the binding site. A key that fits the lock can turn it; a key that fits but cannot turn it just sits there and blocks other keys. That single image already contains the difference between a drug that switches something on and a drug that gets in the way.
Real binding is gentler than a metal key in a metal lock. Proteins flex, and the drug usually nestles in by way of many weak forces working together — hydrogen bonds, electrostatic attractions, the snug fit of greasy surfaces. Because the forces are weak and reversible, the drug binds and unbinds again and again; at any instant only a fraction of the targets are occupied. This in-and-out dance is exactly what later guides will measure with Kd.
Why the right target matters
The body has thousands of different proteins, and many look alike. A good drug binds the one we want and ignores the rest — this is what makes a medicine helpful instead of harmful. The plainest way to name what a drug does at its target is its mechanism of action: which target it grabs and what change it causes there. When you can state a drug's mechanism in one sentence, you understand the most important thing about it.