The simplest way requires NO additional installations: just add a single option to your rails server (or rails s) command when you start up the server:
rails s --binding=0.0.0.0
The 0.0.0.0 address means “listen to requests from anywhere.” On many systems, the default is 127.0.0.1, which means “listen to requests from localhost only.”
(If you don’t also specify a -p or --port option, then the port shall be 3000, as usual.)