What is a restriction in a set?

I am doing self study on graph theory and they use sets. I am reading a paper about how to find patterns in graphs. And they define a sub-graph using sets.

Graph G = (V, E, L) where V, E, L are sets of verticies, edges and lables.

G' = (V', E', L') is a sub-graph if:

V' is a sub set of V

E' is the set E intersect (V' x V')

All this is fine and i get that.

But: L' is a restriction of L on V' U E'

2 questions:

What is a restriction in set theory?

How do I union V' and E' as V is a set of verticies {1, 2, 3, 4} and E' is a set of oredered pairs { (1,2), (1,3), (2,3), (2,4)}

Kind regards

Richard.