What do coffee roast levels mean, and is dark roast stronger?

Roast level describes how long and how hot the beans were roasted, and no — dark roast is not stronger. It contains slightly less caffeine by bean, which is the opposite of what almost everyone assumes.

What happens during roasting. Green coffee beans are largely flavourless. Heat drives off moisture, triggers the Maillard reaction and caramelisation, and develops hundreds of aromatic compounds. The bean expands, loses mass and darkens.

First crack is an audible popping as moisture vaporises and the bean expands — the point at which coffee becomes drinkable. Second crack, later and hotter, is the cell structure fracturing, and oils begin to appear on the surface.

The levels, roughly:

Light. Stopped shortly after first crack. Retains most of the bean's origin character — acidity, fruit, floral notes — because less has been altered by roasting. Higher perceived acidity, and dry-surfaced beans.

Medium. Balanced, with origin character and roast character both present. The most common commercial style.

Dark. Taken to or past second crack. Roast character dominates — smoky, bitter, chocolatey notes that come from the roasting itself rather than from the coffee's origin. Oily surface. Because roasting flavour overwhelms origin flavour, dark roasting is also how poor-quality beans are made palatable, which is part of why it became widespread.

The caffeine question. Caffeine is remarkably heat-stable, so roasting destroys very little of it. But dark-roasted beans lose more mass and expand more, so:

Measured by weight, light and dark are close to identical, with light very slightly ahead.

Measured by volume — scooping beans — dark roast has less, because the beans are larger and lighter.

The perception of strength comes from the bold, bitter, roasted flavour, not from caffeine or from extraction.

What actually determines caffeine in your cup: the coffee-to-water ratio, the brewing method, and the variety — robusta has roughly twice the caffeine of arabica.

Freshness matters more than level. Coffee is best within a few weeks of roasting, and dark roasts stale faster because their surface oils oxidise.