Note : Although the question has been asked quite a time ago, but it may be referred by someone else.
Pre and Post values in a Depth First Search depict the start time of the visit and end time of visit of a vertex respectively. By start time, I mean the time when the vertex is discovered and end time means the time when all the children (in the DFS tree) will be visited.
Here's a sample pseudocode for DFS-
dfs(Graph, Vertex)
static count = 1
pre[Vertex] = count++
visited[Vertex] = true
for all v in Edge(Vertex, v)
if visited[v] = false
dfs(Graph, v)
post[Vertex] = count++;
The pre and post values have a lot of significance. Edge classification is one such example. Also you can find the use of post values where sources and sinks come into picture.