LimeSurvey sends you messages about your survey site, for example after a database update or when you use the theme editor for the first time. They collect under the bell in the top bar, next to your name. The number on the bell counts the messages you haven't read.
Click the bell in the top bar.
The list opens. Unread messages are marked with a dot, read ones are grey.
Delete all notifications at the bottom of the list removes all messages at once.
No. Notifications are only shown to you in the administration.