PostgreSQL is an advanced Object-Relational database management system (DBMS)
that supports almost all SQL constructs (including transactions, subselects and
user-defined types and functions). The postgresql package includes the client
programs and libraries that you'll need to access a PostgreSQL DBMS server.
These PostgreSQL client programs are programs that directly manipulate the
internal structure of PostgreSQL databases on a PostgreSQL server. These client
programs can be located on the same machine with the PostgreSQL server, or may
be on a remote machine which accesses a PostgreSQL server over a network
connection. This package contains the client libraries for C and C++, as well
as command-line utilities for managing PostgreSQL databases on a PostgreSQL
server.

If you want to manipulate a PostgreSQL database on a remote PostgreSQL server,
you need this package. You also need to install this package if you're
installing the postgresql11-server package.