Every once in a while your WordPress website may send you an email with the subject “Your Site is Experiencing a Technical Issue”.
While this may seem scary it is often not a major concern and regularly requires no action to be taken. The email is alerting you that something has happened on your site that was unexpected, in a similar way to when an error message pops up on the screen on your computer.
The second line highlights which plugin caused an issue, this will often hint at where you need to check your site, i.e. if it says “Contact Form 7” you would know to have a look at your contact forms and check if they were working as expected.
Often you will be able to find no issues with your site, if this is the case you should ensure that all of your plugins are up to date and make a note of the plugin that caused the error. If this keeps happening with the same plugin and no software updates are available you may want to consider switching our the plugin for something similar.
If you do find an error in the site, you would need to trouble shoot the error as with any other problem you found on your site.