Never add an inbound rule for 3306 from 0.0.0.0/0, not even "just to test from home". Connect from the EC2 instance in the VPC, or through the bastion in Chapter 15. Public access stays No.
A parameter group holds engine settings, the things you would have put in my.cnf (max_connections, timeouts, character set). Some changes apply immediately. Some wait for a reboot. An option group turns on optional engine features, for example an audit plugin or extra SQL Server options. This MySQL lab is fine on the default groups. Create your own group before you change anything, because the default group is not fully editable.
On the Monitoring tab, watch CPU utilization, database connections and free storage. Free storage heading toward zero is an outage in slow motion. Increase allocated storage before it hits the limit. Autoscaling was off on purpose in this lab. Enhanced Monitoring adds operating-system metrics (memory, processes) at a finer interval. It can add a charge, so read the note on the console before you enable it.