Think of low[v] as how far back you can reach from using the edges in your subtree. If low[v] < disc[v], then there is a back edge from the subtree of to an ancestor. This back edge provides an alternate path, preventing bridges.
If low[v] = disc[v], then the subtree of is a dead end with no back edges escaping. The edge from the parent to is critical because it is the only connection.