How does this happen?

Hi all,
I'm a novice web dev guy at best so please be gentle.
I've noticed that if I insert a hyperlink to an article on some web page into an app, say a whatsapp text message for instance, what gets inserted into the message is not just the link, but a summary, sometimes with images, of the page that the link points to.
I have NO IDEA how this happens, but I'd like to. Can someone give me the lowdown on what's going on here, or point me to something I can read that will clue me in?

Thanks,

keith