In order to connect interactively or via an application to a PostgreSQL database, you need to have login credentials. In particular, a database user, a user who is allowed to connect to that specific database, must exist.
Database users are somewhat similar to operating system users: they have a username and a password (usually encrypted) and are known to the PostgreSQL cluster. Similarly to operating system users, database users can be grouped into user groups in order to ease the massive administration of users.
In SQL, and therefore even in PostgreSQL, both concepts of a single user account and a group of accounts are encompassed by the concept of a role.
A role can be a single account, a group of accounts, or even both depending on how you configure it; however, in order to ease management, a role should express one and only one concept at a time: that is, it should be either a single user or a single group, but not both.