Frozen indices are those indices which are rarely accessed. So, the users freeze these indices. Such indices are called frozen indices. Basically, we freeze those indices which are not in use to free up the memory.
Frozen indices become read-only and its resources are no longer kept active. So, these indices are searchable, but to write again, we have to unfreeze them. Elasticsearch offers an ignore_throttled parameter, which is used to include the frozen indices in your search. Thus, we don't need to re-open them to make available for search.