help me implement Dijkstra's Algorithm

The pseudo code of dijkstra's algorithm is

Code:

` function Dijkstra(Graph, source):`

2 for each vertex v in Graph: // Initializations

3 dist[v] := infinity // Unknown distance function from source to v

4 previous[v] := undefined // Previous node in optimal path from source

5 dist[source] := 0 // Distance from source to source

6 Q := the set of all nodes in Graph

// All nodes in the graph are unoptimized - thus are in Q

7 while Q is not empty: // The main loop

8 u := vertex in Q with smallest dist[]

9 if dist[u] = infinity:

10 break // all remaining vertices are inaccessible from source

11 remove u from Q

12 for each neighbor v of u: // where v has not yet been removed from Q.

13 alt := dist[u] + dist_between(u, v)

14 if alt < dist[v]: // Relax (u,v,a)

15 dist[v] := alt

16 previous[v] := u

17 return dist[]

It's from wikipedia.

I have several clueless area here.

Firstly i like to know how to implement line numbe 4

Code:

` previous[v] := undefined // Previous node in optimal path from source`

what does it mean?

Please help me.