What do the IP addresses(under source column) represent/or mean in Inbound and Outbound rules in AWS security groups?

In your case the IP address in source basically means "from where" the database (I am assuming from port 5432 that it is PostgreSQL database) can be accessed OR from what addresses the "inbound" traffic is allowed. Also in your configuration source address is a set of IPs or subnet. There are other possible options which you can lookup in official docs [here][1].