that is launched when you login
Ordinarily when a shell is invoked it behaves as a standard shell, but if the shell is invoked with a name that starts with `-' (eg. as when started by login(1)), the shell runs as a login shell.
All shells execute the user's shell configuration files, but login shells also execute specific login configuration files, and on exit, execute the logout configuration files.
Typically, the login file contains commands to specify the terminal type and environment.