Troubleshooting Mode was initially a feature introduced via the Health Check plugin, and is now available as a standalone plugin.
The most common, and reliable, means of troubleshooting a website using WordPress, is by first disabling all plugins, and using one of the default themes that come with the software.
The best way to do this, is by the use of a staging website, but not all website hosting providers have this feature, or it may not be part of the plan you are currently on.
Troubleshooting Mode will do its best to allow a site maintainer to disable plugins, and switch themes, without affecting regular visitors to your website.