So you might have seen popup message on some website, here is a simplest way to make that
So to make this I have created a div with id Modal-1, using position property in css fixed the position at the center and then set the display to none so that it's hidden by default.
I have changed the background and given the modal a frost glass effect.
You can try the frost glass effect, check this out : Frost Glass Effect
To Make the popup visible I changed the elements style display to block this will make the modal visible, to make the close button work again change the style.display to none.