Thickness is a property of objects that simulates surfaces in AutoCAD LT.
The thickness of an object is the distance that object is extruded, or extended, above or below its location in space. Positive thickness extrudes upward in the positive Z direction; negative thickness extrudes downward (negative Z). Zero (0) thickness means no extrusion. The Z direction is determined by the orientation of the UCS at the time the object was created. Objects with thickness can be shaded and can hide other objects behind them.
Thickness changes the appearance of circles, lines, polylines (including spline-fit polylines, rectangles, polygons, boundaries, and donuts), arcs, 2D solids, and points. Modifying the thickness of other types of objects does not affect their appearance.
You can set the thickness for new objects you create in AutoCAD LT with the THICKNESS system variable. Change the thickness of an existing object with Properties (DDMODIFY). AutoCAD LT applies the extrusion uniformly on an object: a single object cannot have different thicknesses for its various points.
You may need to change the 3D viewpoint to see the effect of thickness on an object.