The amount of time that a binary log replica DB cluster running on Aurora MySQL-Compatible Edition lags behind the binary log replication source. 
A lag means that the source is generating records faster than the replica can apply them.
The metric value indicates the following:

A high value: The replica is lagging the replication source.
0 or a value close to 0: The replica process is active and current.
-1: Aurora can't determine the lag, which can happen during replica setup or when the replica is in an error state