Browsers don't have the require method defined, but Node.js does. With Browserify you can write code that uses require in the same way that you would use it in Node.

Browserify is a tool for compiling node-flavored commonjs modules for the browser. You can use browserify to organize your code and use third-party libraries even if you don't use node itself in any other capacity except for bundling and installing packages with npm. The module system that browserify uses is the same as node, so packages published to npm that were originally intended for use in node but not browsers will work just fine in the browser too. Increasingly, people are publishing modules to npm which are intentionally designed to work in both node and in the browser using browserify and many packages on npm are intended for use in just the browser. npm is for all javascript, front or backend alike.