A mesh is a set of planar (flat) polygons that approximately represents the shape of a surface, the amount of polygons used in a mesh will directly affect the accuracy of the mesh's shape and file size. A larger amount of polygons will give a more accurate representation of the true shape than would a smaller amount of polygons. It is important that meshes created for IMVU have a low poly count in order to keep the file size down to a minimum to allow for faster download times during chat. In most cases smoothing can be applied to the mesh to compensate for the low poly count. The products you see in imvu are built by developers who specialize in 3d Modeling and are commonly called meshers.