Discovery time disc[v] is the timestamp when DFS first visits vertex . The first vertex visited gets disc[v] = 0, the next gets , and so on. Discovery times create a total order on vertices based on when DFS reaches them. You will use disc[v] to detect ancestors in the DFS tree.
If disc[u] < disc[v], then is an ancestor of (discovered earlier). This helps you identify back edges that point to ancestors.