This symbol represents flatness.

Explanation:

Geometrical tolerance:

• Geometrical tolerance is the maximum permissible overall variation of form or position or measurements of a feature.
• The geometrical tolerance consists of a frame which contains a symbol, representing the geometrical tolerance zone.

Recommended symbols for geometrical tolerancing:

• Geometrical tolerances are arranged into three groups. They are tolerances of form, attitude and location.

Form:

• Straightness, flatness, roundness, cylindricity and profile of a line and a surface.

Attitude:

• Parallelism, squareness and angularity.

Location:

• Position, concentricity and symmetry.