Where flat design removed shadow entirely, material design kept one metaphor — paper has real thickness and can stack in layers — and flattened everything else. Its core rule is expressing each layer's height (elevation) through shadow size and spread: 1dp gets a faint shadow, 8dp a larger, more diffuse one.
Its signatures also include the ripple that spreads from a touch point, shared-element transitions that carry a piece across screens, and the FAB (floating action button). The 2021 "Material You" update added dynamic color, pulling a palette from the user's wallpaper.
It's the default for Android apps and Google's own products, and its rules — spacing, type scale, component states — are documented in enough detail that a whole team can stay in sync using them.
When to use
Good for Android apps and admin dashboards with many components and larger teams. For an iOS-only app, following Apple's HIG matches user expectations better.