test1.js

// FILE - 1
function greet() {
  console.log('Hello');
}

module.exports = greet;
// this is the way to export a function 
// from a module
// module means file

test2.js

var myGreetFunction = require('./test.js');
// this basically means loading a file
// and assigning the exported variables 
// to a variable in this file
// in this case, the exported variable is a 
// function
console.log(myGreetFunction);

node test2.js

EDIT: Change require('./test.js'); to require('./test1.js');