What does they signify and how to read them
As the name suggests, FreeStorageSpace means it's your RDS's storage(hard disk).
While Freeable memory
is your RAM
. If this is always low, upscale your RDS or optmize your application.
You can create alarms for both in cloudwatch. You can find both metrics while making alarms in CloudWatch->Create Alarm->Select Metric->RDS->Per-Database Metrics->FreeableMemory/FreeStorageSpace
for the respective DB.
The alarms could be if FreeStorageSpace<20GB for 15 min
send an email or if FreeableMemory <= 200MB for 15min
send an email.
This is an overview, you can read more about creating alarms and setting up SNS
here.