What does star (*) mean in JavaScript function definition in Koa framework? [duplicate]

I have been familiarising myself with Koa (http://koajs.com/). Many of the examples include star character in place of function name. For instance in the hello world example there is:

var koa = require('koa');
var app = koa();

app.use(function *(){
  this.body = 'Hello World';
});

app.listen(3000);

What does this star mean?