What is the Maintenance DB asked by pgAdmin?

It is worth mentioning that you are free to define any existing database in the cluster as "Maintenance DB". You are not limited to the options suggested by the interface, which are just typical defaults.

This is particularly useful for users with limited permissions or read-only access. Access to a single DB is simpler with the same DB as "Maintenance DB". Related:

• Do all users need access to the postgres database in pg_hba.conf?

What is the maintenance db? The manual:

The postgres database is also created when a database cluster is initialized. This database is meant as a default database for users and applications to connect to. It is simply a copy of template1 and can be dropped and recreated if necessary.

From the docs:

The maintenance DB field is used to specify the initial database that pgAdmin connects to, and that will be expected to have the pgAgent schema and adminpack objects installed (both optional). On PostgreSQL 8.1 and above, the maintenance DB is normally called ‘postgres’, and on earlier versions ‘template1’ is often used, though it is preferrable to create a ‘postgres’ database for this purpose to avoid cluttering the template database.

I'd add that once you are connected, it will give you list of other databases on that server you can also connect to. But until you connect to at least one database on the server, you can't fetch that list.

If it was just called "DB", that would suggest it is the only database on that server you can connect to, rather one of (potentially) several.