The login shell executes /etc/profile if this file exists and is readable. This file does the shell setup that applies to all users, so you'll be interested in it if you're the system administrator; you need to log in as root in order to edit it.

The /etc/profile file:

If you have a script that you want to run whenever anyone on the system runs a login shell, put it in the /etc/profile.d directory. You must have root-level privileges to add a file to this directory.

For example, if you need to set global environment variables or run certain tasks when anyone logs in, then this is the place to put a script to handle it. If you're using sh as your login shell, make sure that the script has a .sh extension.