Handlers in web application

I am new to web app development. The pyhton file which displays 'Hello World' in the browser is

import webapp2

class MainHandler(webapp2.RequestHandler):
    def get(self):
        self.response.write('Hello World')

app = webapp2.WSGIApplication([('/', MainHandler)
], debug=True)

What I couldn't understand is the function of the line

app = webapp2.WSGIApplication([('/', MainHandler) ], debug=True)

I have searched the net for the answer but failed to get a satisfactory answer. It would be great if anyone would explain it considering I am a beginner.