The abbreviation "motd" stands for "message of the day", and this file has been traditionally used for exactly that (it requires much less disk space than mail to all users).

The contents of /run/motd are displayed by login after a successful login but just before it executes the login shell.

- Ubuntu Manpage for Message of the Day

The /run/motd file is a text file with contents that can be displayed to all users logging in on the system. The content of the file is usually a system message, which can be used to convey information about the current state of the system to users logging in.

Message of the Day prompt on Window Terminal window

Execute the following command on a Terminal window to display the content of /run/motd;

cat /run/motd

This file is used to display system information, to alert users of system-wide changes, or to advertise information. In some cases, system administrators might want to display specific message to users after successful login.

Content of the /run/motd file show on Window Terminal window

To disable the "message of the day" prompt, execute the following command on a Terminal window and create a blank .hushlogin file at the root folder of the current user;

touch ~/.hushlogin