Basically, mesh is a collection of vertices, edges and faces represent the shape of an object in 3D space, which is heavily used in both real life and computer graphics.
For a real world example, imagine that you have a building, then the mesh would be its steel frame structure.
In computer graphics, you may have seen a lot of 3D model used in gaming project or in film. They are all constructed from mesh:
There are 2 type of mesh: Polygonal and Volumetric. Polygonal meshes only represent the object surface, such as simple geometry like triangle, rectangle or a plane, while Volumetric meshes represent both the surface and the volume of the object.