What is the meaning of inbound source of default RDS security group

Question

When I create an RDS, it seems to have an inbound source by default.

For example, like port: 5432, IP: 221.142.31.25/32.

As I understand, It means that the security group allows requests from the IP range(221.142.31.25/32) to access the port(5432).

Somehow It seems to allow my local to access the RDS as well without any additional inbound source representing my local.

In contrast to my local, when I try to have access to the RDS from Lambda, I have had to add inbound source 0.0.0.0/0, otherwise, the Lambda has returned a timeout error.

My question is...

1. What does the IP range(221.142.31.25/32) mean?

2. How does it allow my local to access the RDS?

3. Why does it deny Lambda but my local?