Every point is a pair of backbone angles (φ, ψ). Drag the marker, or pick a preset, and see which backbone conformation those two numbers force the chain into — and whether geometry even allows it. The shaded islands are the empirically favoured regions.
Regions are a schematic of the classic favoured basins from Ramachandran et al. (1963); the exact contours depend on residue type and force field. φ is rotation about N–Cα, ψ about Cα–C. In glycine mode the missing side chain opens up the mirror-image (positive φ) regions that are forbidden to every other residue. Angles run −180° to +180° on both axes.