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.
If is an ancestor of in the DFS tree, then disc[u] < disc[v]. But the reverse is not always true. A node discovered earlier might be in a different subtree, not an ancestor. To confirm ancestry, you also need the finish time: is an ancestor of if and only if disc[u] < disc[v] < fin[u].