This read-only value reports the Forward Delay time advertised by a specific MST instance's root bridge, which governs how long ports carrying that instance's VLANs take to reach forwarding after a topology change. An admin checks it per instance since different instances can, in principle, converge at different rates depending on their own root's settings. For example, seeing a longer-than-default Forward Delay here for instance 2 would explain why VLANs mapped to that instance recover more slowly after a failover than VLANs in instance 1.