Drupal is a popular CMS built in PHP, which is flexible and powerful. Currently in version 7 and the next version 8 is due in some time. Drupal has a way to Log and Report Errors. Like almost all CMS, it also periodically checks for new updates to the core, as well as themes and modules. Once an update is detected, it will Log Report it. It will also report about other errors in your code.
This all helpful in making your there are no bugs and security holes left for your site to be hacked. However you would not like the visitors and viewers of your site to have warning messages shown at the top of every page. To solve this issue, Drupal has a way to Turn them Off for the visitors and let it remain On for the admin user.
Just login to your site as admin and navigate to Configuration (at the top), then under Development click on Logging and errors. You can directly go to it by pasting this URL segment at the end of your drupal home page URL: admin/config/development/logging
You will see a screen similar to the picture below. Just select None and click on Save configuration to make it effective.