Runtime configuration can be used to modify various server settings without restarting Envoy. The runtime settings that are available depend on how the server is configured. They are documented in the relevant sections of the configuration guide.
Runtime guards are also used as a mechanism to disable new behavior or risky changes not otherwise guarded by configuration. Such changes will tend to introduce a runtime guard that can be used to disable the new behavior/code path. The names of these runtime guards will be included in the release notes alongside an explanation of the change that warrented the runtime guard.
Due to this usage of runtime guards, some deployments might find it useful to set up dynamic runtime configuration as a safety measure to be able to quickly disable the new behavior without having to revert to an older version of Envoy or redeploy it with a new set of static runtime flags.