Number of seconds a Replica Server is allowed to fall behind the Primary Server.
In asynchronous and semi-synchronous replication, Replica Servers can fall behind the Primary Server, due to limitations of hardware or workload. This is called replication lag. Lagging Replica Servers do not provide current data to clients executing read queries.
This module parameter controls the amount of replication lag the router permits. If a Replica Server falls too far behind, the router stops routing client connections to it. This is to allow the Replica Server to catch up with the Primary Server and to prevent it from serving old data to clients.
Information specific to MariaDB SkySQL can be found on the max_slave_replication_lag page in the SkySQL Documentation.