• The Postgres installer asks for your usual Mac admin account password, to get permission for 2 operations: (1) to install stuff on your computer and (2) to create a special Unix user account named (by default) 'postgres'.
The actual disk files that make up your databases, the files containing your data, are stored in a folder owned by that 'postgres' user rather than your normal Mac user account. The reason is security: If your usual user account is compromised, at least your data files remain behind the wall of that other user.
• The Postgres installer also prompts you to create a password for that 'postgres' user.
Make it a good password (long, use digits etc. to avoid being simply dictionary words).
When you run the 'pgAdmin' app, it will prompt you for that 'postgres' password to connect to the database server as that 'postgres' user.