Maduro means “ripe” in Spanish. On a cigar band it usually means a dark brown to near-black wrapper that has gone through more fermentation (and sometimes more time and heat) than a natural leaf from the same crop. The word describes a process, not a seed.
A darker wrapper is not automatically stronger. A maduro wrapper can taste sweet, earthy, or mild; the filler still determines most of the strength. Companies such as Padrón sell the same blend with natural and maduro wrappers, so you can taste the difference the extra fermentation makes to the wrapper.
Not every dark wrapper is a maduro in the factory sense, and not every maduro is an oscuro. Shops often use the words loosely. This page uses maduro to mean a wrapper that was deliberately darkened through longer fermentation.