Use the Flatten Faces tool to force two or more object faces into the same plane.
When two or more object faces are selected, you can select the Flatten Faces tool from the Context menu. You can also access Flatten Faces using the shortcut "FL."
Flatten Faces can be useful as a repair tool when a face that should be planar becomes unexpectedly faceted (with smooth edges between the facets), either when importing lower-fidelity data like SketchUp models, or after accidentally moving a vertex or edge which causes a face to get faceted or triangulated.
You can also use Flatten Faces as a design tool for aligning or extending shapes. To do this, first select a guide surface, then select additional faces to flatten to the plane of the first surface.
Check out the video below to see the power and versatility of Flatten Faces: