A server administrator can define one or more system maintenance messages. These are displayed when signing in, as pulse notifications (if more than one message has been configured, each is shown in turn; notifications are not sent if system configuration setting EnablePulseMessages is set to False).
System maintenance messages have start and expiry dates, between which they are active. Only active system maintenance messages are displayed.
Active system maintenance messages are also shown if added by the server administrator when you are already signed in (similarly, messages are shown if they become active when you are signed in).
Full details of pulse notifications can be found elsewhere in the documentation.