Do not defragment a single index more than this number of time per second.This controls the number of time defragmentation thread can request X_LOCK on an index. Defragmentation thread will check whether 1/defragment_frequency (s) has passed since it worked on this index last time, and put the index back to the queue if not enough time has passed. The actual frequency can only be lower than this given number.
Additional information on this topic may be found in the MariaDB Public Knowledge Base.
Information specific to MariaDB SkySQL can be found on the innodb_defragment_frequency page in the SkySQL Documentation.