Mesh is an option for SurfaceGraphics and DensityGraphics that specifies whether an explicit - mesh should be drawn.
See the Mathematica book: Section 1.9.6, Section 1.9.7, Section 2.9.7, Section 2.9.11.
See also: FaceGrids, Boxed.
A density plot with and without a mesh.
Evaluate the cells to compare the graphics.