From BlenderWiki

This panel appears in the Editing Context panel group which is accessed using F9 or clicking button in the Buttons window.

[edit] Mesh Tools 1

• Centre - Moves the vertices altogether so that their barycenter is set to the Object center.
• Hide - All selected vertices are temporarily hidden (H).
• Select Swap - Toggle the selection status of all vertices.
• Reveal - This undoes the Hide option (Alt H).
• Nsize: - The length of the face normals, if they have been drawn.
• Draw Normals - Indicates that the face normals must be drawn in EditMode.
• Draw Faces - Indicates that the face must be drawn (as shaded blue/purple) in EditMode.
• Draw Edges - Indicates that the selected edges must be highlighted. Partially selected edges (one vertex only selected) have a color gradient.
• Draw Creases - Display creases for sub-surface weighting.
• Draw Seams - Display UV unwrapping seams.
• All Edges - After leaving Edit Mode, all edges are drawn normally, without optimization.
• Draw VNormals - Display vertex normals as lines.
• Edge Length - Display selected edge lengths.
• Edge Angles - Display the angle in the selected edges in degrees.
• Face Area - Display the area of selected faces.