The 3D modeling software that you use to create your design may offer you a way to export it to .stl. STL stands for standard triangle language and means that your design will be translated into triangles in a 3D space. Not all modeling software however offers this approach. It may instead provide a description of your design as a mathematical function. For example, with most CAD software, a sphere is described as an object with a specific diameter or radius. With STL, your sphere will be described as a composition of triangles.