The BFS Algorithm Step 3

The BFS loop runs while the queue is not empty:

while queue is not empty:
    u = queue.pop_front()
    for each neighbor v of u:
        if not visited[v]:
            visited[v] = true
            dist[v] = dist[u] + 1
            queue.push_back(v)

Each iteration removes the front node $u$ from the queue. Examine every neighbor $v$ of $u$. If $v$ is already visited, skip it. If $v$ is unvisited, mark it visited, set dist[v] to dist[u] + 1, and add $v$ to the queue.

Repeat until the queue is empty. The distance array now holds the shortest distance to each node.