kiki...Yellowat?
Welp, I kinda did a research (actually just searched "Annotations" on the search bar) and found this extremely detailed guide by Michael
"How to Add Annotations On YouTube Videos - Annotations Guide" on the "YouTube Tips, Tutorials, Help & Guides" section
"Label - Labels are less common but are very useful when you want to label parts of your video for example if you had several pictures that were shown, you might wish to label them using the label annotation. The label looks like a spotlight annotation until hovered which reveal the text label." -> Taken from the post
is what came closest to what I thought you would be asking, thought it looks different.
It still got the same concept.