Update index.md

Co-Authored-By: Pietro Monticone <38562595+pitmonticone@users.noreply.github.com>
Co-Authored-By: Claudio Moroni <43729990+ClaudMor@users.noreply.github.com>

Author: 3 people

docs/src/index.md

@@ -34,6 +34,8 @@ Multilayer graphs have been adopted to model the structure and dynamics of a wid
 
 MultilayerGraphs.jl is an integral part of the [JuliaGraphs](https://github.com/JuliaGraphs) ecosystem extending [Graphs.jl](https://github.com/JuliaGraphs/Graphs.jl) so all the methods and metrics exported by Graphs.jl work for multilayer graphs, but due to the special nature of multilayer graphs the package features a peculiar implementation that maps a standard integer-labelled vertex representation to a more user-friendly framework exporting all the objects an experienced practitioner would expect such as nodes ([`Node`](@ref)), vertices ([`MultilayerVertex`](@ref)), layers ([`Layer`](@ref)), interlayers ([`Interlayer`](@ref)), etc.
 
+A multilayer graph is composed of layers, i.e. graphs whose vertices represent the same set of nodes (not all nodes need to be represented in every layer), and interlayers, i.e. the bipartite graphs that connect vertices in two different layers. Vertices in a multilayer graph are represented using the `MultilayerVertex` struct, while nodes are represented using the `Node` struct.
+
 `MultilayerGraph` and `MultilayerDiGraph` are fully-fledged Graphs.jl extensions. Both structs are designed to allow for layers and interlayers of any type (as long as they are Graphs.jl extensions themselves) and to permit layers and interlayers of different types. However, it is required that all layers and interlayers in `MultilayerGraph` are undirected, and all layers and interlayers in `MultilayerDiGraph` are directed.
 
 `MultilayerGraph` and `MultilayerDiGraph` support the specification of vertex and edge metadata, provided that the underlying layer or interlayer also supports metadata.