Forever is a npm package, it is for your node.js server side application. Look this package document. forever npm package.
Use forever so that eventhouth you log off your machine, the node.js will still running the application for you. Without this, after you log off, your application will logged off.
Few very command command for running at Linux System.
list all the forever running thread
/# forever list
stop specific forever running thread
/# forever stop server.js
start specific forever running thead
following example is similar to node server.js
/# forever start server.js