Meshes are the central feature of any 3D engine. Meshes can be separated into two kinds: the ones used for rendering, and the ones used for interactions (physics).
Mesh data is either imported from external files, such as FBX files, or procedurally generated. Generating meshes procedurally is mostly useful for very basic shapes either for special cases or as placeholders during early development.
Once a mesh is imported as an asset, it can be placed in a scene as often as you like. For the most common use case you would use a mesh component to do so, but there are other components for special cases, such as the instanced mesh component. Meshes may also be used by other things, for example as a type of particle. To instantiate collision meshes, you need to use the proper shape component.