While BFS is applied, the vertices of the graph are divided into two categories. The vertices, that are visited as part of the search & those vertices that are not visited as part of the search. The approach adopted in breadth first search is to begin search at a vertex(source). Once you begun at source, the number of vertices that are visited as part of the search is 1 and all the remaining vertices need to be visited.
Then, search the vertices that are adjacent to the visited vertex from left to order. In this way, all of the vertices of the graph are searched.
Let the digraph of Figure. Assume that the search begun from S. Now, the vertices (from left to right) adjacent to S that are not visited as part of the search are B, C, A. So, B,C and A are visited after S as portion of the BFS. Then, F is the unvisited vertex adjacent to B. hence, the visit to B, C & A is followed by F. The unvisited vertex adjacent of C is D. Thus, the visit to F is followed by D. There are no unvisited vertices adjacent to A. At last, the unvisited vertex E adjacent to D is visited.
So, the sequence of vertices visited as part of BFS is S, B, C, A, F, D and E.