It starts at the tree root, and explores all of the neighbor nodes at the present depth prior to moving on to the nodes at the next depth level. code example

Example 1: bfs

function breadthFirstSearch (Start, Goal)
   { 
       enqueue(Queue,Start)
       setVisited(start)
       while notEmpty(Queue)
       {
           Node := dequeue(Queue)
           if Node = Goal
           {
               return Node
           }
           for each Child in Expand(Node)
           {
               if notVisited(Child)
               {              
                   setVisited(Child)
                   enqueue(Queue, Child)
               }
           }
       }
   }

Example 2: bfs algorithm

function breadthFirstSearch (Start, Goal)
   { 
       enqueue(Queue,Start)
       setVisited(start)
       while notEmpty(Queue)
       {
           Node := dequeue(Queue)
           if Node = Goal
           {
               return Node
           }
           for each Child in Expand(Node)
           {
               if notVisited(Child)
               {              
                   setVisited(Child)
                   enqueue(Queue, Child)
               }
           }
       }
   }