The "Information Box" on a cover photo

Without displaying a pic of what I am trying to say I will hopefully convey this correctly.

When you hover your mouse over a cover photo, you will eventually get the word in a yellow "information box" that says cover photo. I have seen other fb pages that have actual information written here. How do I do this?

Here is a url of what I am trying to say. (Hope this forum let's us use urls) Yes, I am new here. lol.

https://www.facebook.com/LearningToLeadDogTraining/photos/pb.299697866763652.-2207520000.1454859904./965726460160786/?type=3&theater