purification - What does it mean for meth or any other compound to be 'pure'?

In Breaking Bad, Walter White can cook $99.1\%$ pure meth. In general what does it mean for a substance to be pure? In this case, what exactly does it mean for meth to be pure? Does higher purity mean less unhealthy? More addictive? Giving more euphoria for the user?

For comparison, is the purity of prescribed methamphetamine or methylphenidate to people with ADHD or narcolepsy close to $100\%$? My guess is that purity is how close Walt's product is with legal meth.