This setting is only used by the forceMerge action, and is optional.

action: forcemerge
description: >-
  Perform a forceMerge on selected indices to 'max_num_segments' per shard
  max_num_segments: 2
  timeout_override: 21600
  delay: 120
- filtertype: ...

The value for this setting is the number of seconds to delay between forceMerging indices, to allow the cluster to quiesce.

There is no default value.