How does grammarly.com make revenue?

I watched youtube video and an ad came out, called Grammarly. https://www.youtube.com/watch?v=VueSBzrIh1o As it said, it provides grammar auto-fix for people. And it said it's better than MS office. So how do you think they make revenue? I saw it's a free Chrome app, actually, no charge... I am wondering how this startup survives. Welcome to comment!