I saw an example of creating tables (see below). I wonder what the primary key (user_name, role_name) in the table user_roles means? Does it mean that both user_name and role_name are the primary key of the user_roles table? How does a table have two primary keys?

create table users ( user_name varchar(15) not null primary key, user_pass varchar(15) not null );

create table user_roles ( user_name varchar(15) not null, role_name varchar(15) not null, primary key (user_name, role_name) );