A Mesh is another term for the net of geometric polygons that make up the objects in your scene. Meshes reference, or are referenced by attribute tags such as surfacing and UV information, animation rigs, simulation and rendering engines. Without meshes, you would have only parametric objects to render. Meshes are formed by vertices and the faces that are created from a minimum of three vertices (triangles). Edges bridge the connection between two or more vertices. You should avoid free-standing vertices, edges or polygons in your mesh, unless said elements are intended to serve a specific purpose.
You may have seen other terms used instead of “mesh” but the other terms all apply to the basically the same definition.