What's the "accent color"?

You got good example in Material Design docs -> your palettes are shown in section "UI color application" - with blue colors being primary and pink being used for accents.

Here is what it means in practice (also FAB would most likely use pink background color)

Quoting from http://android-developers.blogspot.se/2014/08/material-design-in-2014-google-io-app.html:

In material design, UI element color palettes generally consist of one primary and one accent color.

The accent color is used more subtly throughout the app, to call attention to key elements. The resulting juxtaposition of a tamer primary color and a brighter accent, gives apps a bold, colorful look without overwhelming the app’s actual content.

Accent color is also known as Secondary color.

It is used for:

• Buttons
• Floating action buttons
• Selection controls, like sliders and switches
• Highlighting selected text
• Progress bars
• Links and headlines