Query Monitor (QM) is a WordPress plugin that provides a developer tools panel for WordPress. It enables debugging of database queries, PHP errors, hooks and actions, block editor blocks, enqueued scripts and stylesheets, HTTP API calls, and more.
The WordPress.org listing for this plugin provides a more complete feature list and a guide for usage:
Query Monitor is available by default on all VIP production and development sites. You can add the
view_query_monitor capability to any role that should have access. Alternatively you can filter
wpcom_vip_qm_enable directly in your query-monitor.php file.
Either method is fine to use, but avoid disabling access for VIP Support. For example, if you filtered
wpcom_vip_qm_enable to only return
true for your team, VIP team members would no longer be able to use Query Monitor on your site in a situation where you require support.
You can also set an authentication cookie which allows you to view Query Monitor output when you’re not logged in (or if you’re logged in as a user without the
view_query_monitor capability). See the bottom of Query Monitor’s output for details.