Since the pg_stat_statements shared library has been loaded by the PostgreSQL server, Postgres starts collecting statistics for every database in the instance.
The extension simply installs the pg_stat_statements view and the pg_stat_statements_reset() function in the current database, allowing the DBA to inspect the available statistics.
By default, read access to the pg_stat_statements view is granted to every user who can access the database (even though standard users are allowed to see only the SQL statements of their queries).
The pg_stat_statements_reset() function can be used to discard the statistics collected by the server up to that moment, and set all the counters to 0. It requires a superuser in order to be run.