subscribe to posts plugin/widget

I have seen many post where there is a space similar to the ‘search’ space and there is a message saying something like:

“Enter your email address to subscribe and receive notifications of new posts by email.”

And, you enter your email and that’s it! You start receiving emails every time a new post is added to the blog.

Can someone point me to this plugin?

I have searched for some time now and can’t figure out what plugin or widget is used or how this is . Thank you.