Face Mode, also known as Polygon Mode, can be accessed by clicking on the Face icon when your 3D mesh is selected. This can also be accessed using the Clara.io keyboard shortcut, Y.
When in Face Mode, you’re granted access to modeling tools from the Tools tab that is aimed primarily at modifying polygonal faces. You can also further manipulate your selected Faces using the Transform Manipulators.
You can learn more about the Face Component Tools here.