The slow query log will record all queries which are above the threshold level. The default value is 10 (seconds) but you can set it higher or lower depending on your requirements. It is useful for finding slow queries and allows you to pick out candidates for tuning.
If you set the threshold too low, it can increase I/O overhead on your instance and use a lot of valuable disk space. If you set it too high, it might not capture enough useful information.
This is a setting in AWS Relational Database Services and mimics functionality in MySQL