Number of hash buckets for open and changed files. If you have a lot of Aria files open you should increase this for faster flush of changes. A good value is probably 1/10 of number of possible open Aria files.
Additional information on this topic may be found in the MariaDB Public Knowledge Base.
Information specific to MariaDB SkySQL can be found on the aria_pagecache_file_hash_size page in the SkySQL Documentation.