How to read the mathematical notation for multigraphs: $$E \rightarrow V \cup[V]^2 $$
$E$ is a set of edges
$V$ is the set of vertices
I am having trouble especially with this part $$[V]^2 $$
A multigraph is a pair $(V,E)$ of disjoint sets (of vertices and edges) together with a map $E →V ∪ [V ]^2$ assigning to every edge either one or two vertices, its ends. Thus, multigraphs too can have loops and multiple edges: we may think of a multigraph as a directed graph whose edge directions have been ‘forgotten’. To express that $x$ and $y$ are the ends of an edge $e$ we still write $e = xy$, though this no longer determines e uniquely.
(Reinhard Diestel, Graph Theory 5th Edition, Springer, p.28 )