By the dimensions listed I assume you are referring to a rectangular prism. This is like a 3D rectangle with a length, width, and depth. Imagine a 3D rectangular solid with a length of 4 cm, a width of 2 cm, and a depth (or "thickness") of 1 cm. The total surface area of the solid is the sum of the areas of all six of the surface faces. So there will be two faces with an area of 4*2=8 cm, two faces with an area of 4*1=4 cm, and two faces with an area of 2*1=2 cm. So the total surface area will be 8+8+4+4+2+2=28 cm.
The surface area is 28 cm.