How do you set the "floating" label on view items?

I'm sorry for not having a better description of the title, cause I'm honestly not sure what to call it. In many applications, you can just place your mouse icon over a button or a text field, or whatever, and it'll pop up with a short description of the button. However, I can't figure out how to add them to my buttons and text fields, so anyone got any advice?

 

That was exactly it, thank you.