A hydrocarbon is a molecule made of nothing but carbon and hydrogen atoms. On their own, hydrocarbons are relatively chemically unremarkable — mostly notable for burning, which is exactly why they make good fuels. What makes organic chemistry as rich and varied as it is has less to do with the hydrocarbon backbone itself and much more to do with functional groups: small, specific clusters of atoms that, when attached to that backbone, give the resulting molecule reliably distinctive chemical behaviour that the plain hydrocarbon never had.
The backbone provides structure; the functional group provides behaviour
A functional group is a specific, recognisable arrangement of atoms within a larger molecule that behaves in a broadly predictable way regardless of what particular hydrocarbon backbone it happens to be attached to. The hydroxyl group, a single oxygen bonded to a hydrogen, turns a hydrocarbon into an alcohol, giving it water-solubility and a much lower flashpoint than the plain hydrocarbon it was built from. A carboxylic acid group makes a molecule mildly acidic. An amine group, built around a nitrogen atom, is present in an enormous range of biologically active molecules, including many drugs, because that nitrogen can participate in the kind of hydrogen bonding and charge interactions that let a molecule interact specifically with a receptor or enzyme in the body. The backbone mostly determines a molecule's size, shape, and general physical properties; the functional group is what actually determines how it reacts and what it does chemically.
Same backbone, wildly different molecule
Because functional groups behave in broadly consistent, well-characterised ways across many different underlying hydrocarbon backbones, organic chemists can predict a great deal about how an unfamiliar molecule will behave just by identifying which functional groups it carries, without needing to work out its full behaviour from first principles every time. This is also why a very small structural change — swapping one functional group for another, or adding one to a plain hydrocarbon that had none — can turn an inert or straightforwardly combustible fuel molecule into something with completely different, sometimes powerful pharmacological activity. Many drugs are, structurally, a hydrocarbon skeleton not too different from ones found in fuels or common industrial chemicals, distinguished mainly by which functional groups have been attached to it and exactly where.
What we're still unsure about
The general principle that functional groups govern a molecule's reactivity largely independent of its specific hydrocarbon backbone is one of the most well-established organising ideas in organic chemistry. What's still an active area of pharmaceutical research, rather than settled knowledge, is precisely predicting how a specific arrangement of functional groups on a specific backbone will behave inside a living biological system — a molecule's chemical reactivity in a test tube doesn't always translate cleanly into predicting how it will be absorbed, metabolised, or how it will interact with the specific complex biological targets in the human body, which is a large part of why drug development still requires extensive testing rather than being predictable from structure alone.
This sits inside Hydrocarbons & Functional Groups, one of eight topics in Organic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.