What is a postgres superuser

https://www.postgresql.org/docs/current/static/sql-createrole.html

These clauses determine whether the new role is a "superuser", who can override all access restrictions within the database.

yes - all privileges in all databases on the specified cluster

A superuser in PostgreSQL is a user who bypasses all permission checks.

Superusers can run commands that can destabilize or crash the database server (e.g., create C functions) and access the operating system.

Superuser is per database. From manual:

SUPERUSER NOSUPERUSER These clauses determine whether the new role is a "superuser", who can override all access restrictions within the database. Superuser status is dangerous and should be used only when really needed. You must yourself be a superuser to create a new superuser. If not specified, NOSUPERUSER is the default.

The superuser role bypasses all permission checks.

The term "cluster" in PostgreSQL is a historical and it is un-related to the PostgreSQL command CLUSTER, which is about organizing tables - you set user per-database, not per-cluster. A cluster is created when PostgreSQL is installed. The installation will usually do initdb - it will create a new cluster with one database in for you. Cluster is just data dir with databases inside. For location of that data dir look into manual.