When we connect to PostgreSQL, we always connect to just one specific database on any database server. If there are many databases on a single server, it can get confusing, so sometimes you may just want to find out which databases are parts of the database server.
This is also confusing because we can use the word "database" in two different but related contexts. Initially, we start off by thinking that PostgreSQL is a "database" in which we put data, referring to the whole database server by just the word "database". In PostgreSQL, a database server is potentially split into multiple individual databases, so as you get more used to working with PostgreSQL, you'll start to separate the two concepts.