Global traits, local components
The idea is to combine the best bit of global styling (reuse, small payload) and local styling (total isolation, first-class React syntax)
This is combined with the concept of traits: you can think of them as permitted property/value pairs. Instead of every component being able to have every CSS property available to it, you can reduce your permitted set to X font families, Y font-size + line-height pairs, Z foreground/background colour pairs, W padding amounts. This is based off my work using amcss on real projects — traits were the single key feature that kept me using AM.
The one-sentence explanation: A site defines a set of permitted visual features, all components are simply a combination of those features