algorithm - Breadth First Search Pseudo Code Understanding -

i have 2 questions regarding pseduo code above.

1. what function solution(node) do? how 1 implement it?
2. on 3rd last line, check if child not in frontier. how 1 check in fifo queue?

1. solution(node) returns full solution problem instead of node. in pathfinding problem, might return full path start end node

example:

def solution(node):      result = []     while(node.predecessor not none):         result.append(node.predecessor)         node = node.predecessor 

2. linear search frontier. if isn't possible, still work long check whether node in explored after dequeued, requires memory.