How do I label photos in iPhoto on my MacBook

Are you using iPhoto or Photos? Your question is in the Photos for Mac forum, but you mention iPhoto?

In Photos for Mac you can add titles, if you select a photo in Photos, then open the Info with ⌘I, then type a titel into the field "Add a Title".

If you go to the menu "View > Metadata" and enable "Titles" you can see the titles below the thumbnails, but only in the albums "Recents"; "Imports"; or any album you create yourself. It will look like this: