Green coffee beans taste of almost nothing. Grassy, a bit like a dried pea. Everything you recognize as coffee happens in the roaster, and roast level is just how far along that process somebody stopped.
The useful way to think about it: the longer you roast, the more the flavor comes from the roasting and the less it comes from the coffee itself. That’s the whole trade.
Light
Stopped early. Tastes most like where it grew — fruit, flowers, citrus, tea. Noticeably tangy, which some people love and some people think means it’s undercooked. It isn’t.
Light roasts are less forgiving to brew. If it comes out thin and sour, that’s usually not the roast’s fault — see why your coffee tastes bitter or sour.
Medium
The middle, and the one most people are happiest with. Some of the origin character survives, some caramel sweetness has developed, and nothing is shouting. If you don’t know what you want, this is the right place to not know it.
Dark
Roasted long enough that the roasting itself is the flavor: chocolate, toast, smoke, a pleasant bitterness. The differences between origins mostly burn off, which is why big blends tend to go dark — it makes the coffee taste the same every time regardless of what went in.
That’s a genuine skill and a genuine service, and specialty coffee spent about fifteen years being unbearably rude about it. Dark roast is also the right answer if you drink your coffee with a lot of milk, because it’s the only one that still tastes of anything afterward.
Two things that aren’t true
Dark roast is not stronger.It tastes bolder, but roasting burns caffeine off rather than adding it, so if anything a light roast has very slightly more. The difference is small enough not to matter, and either way it’s the opposite of what the packaging implies.
Dark roast is not a way of hiding bad coffee.It can be, and cheap producers do use it that way. But plenty of excellent roasters take good beans dark on purpose because that’s the drink they want.
What actually matters more
When it was roasted. A medium roast from three weeks ago will beat any roast level from six months ago, every time, and it isn’t close. Roast level is a preference; freshness is a fact. That argument is over here.