While I was reading Reinhard Diestel text on graphs, I came across this paragraph.
Let G = (V, E) and G' = (V' , E' ) be two graphs. We call G and G' isomorphic,and write G $\simeq $ G' , if there exists a bijection φ: V → V with xy ∈ E $ \leftrightarrow $ φ(x)φ(y) ∈ E for all x, y ∈ V . Such a map φ is called an isomorphism; if G = G' , it is called an automorphism.
We do not normally distinguish between isomorphic graphs. Thus, we usually write G = G' rather than G $ \simeq $ G , speak of the complete graph on 17 vertices, and so on.
A map taking graphs as arguments is called a graph invariant if it assigns equal values to isomorphic graphs. The number of vertices and the number of edges of a graph are two simple graph invariants; the greatest number of pairwise adjacent vertices is another.
What does this paragraph mean and what are graph invariants, in simple explanations.