Fig 1 is Graph G
Fig 2 is BFS done on graph G
Bold Colour Edges in fig 2 are BFS Tree edges.
At Right side, instances of Queue 'Q' is derived.
Observe at instance no. 4 ., distance from source 's' to 'w' & 'v' is 1 & 2 respectively.
similarly at instance no 7, distance from source 's' to 'x' & 'u' is 2 & 3.
Therefore, v2.d <= v1.d + 1