If you add the link to an external web page in your post, a preview of that page will be generated. Once you write the link, you'll have to wait a few seconds for it to load. The appearance of an icon on the right side of the attachments area will let you know that the preview ready. You will be able to see how it will look by tapping on this icon.
You can add more links, but only the first one will generate a preview.
It will always appear at the end of your publication, after the GIF, audiovisual content and text (always in this order) that you may have wanted to include.
Just tap on the preview to visit that web page. But remember to go back to Launchyoo! :).