To display an element on-mouse-over of another element – because that’s actually what it does, wether the displayed element is on top of the mouse-overed one or some place else – like the tutorial video shows:
Create an element (say it’s you thumbnail), create another element of the same size (or both sizes 100%) as a child and give it a color, reduce the opacity and put your desired text in. Give the parent element position:relative and the child position:absolute, fine tune to get one on another. Give a unique class to each, then give the child the property display:none or opacity 0%. Create an interaction saying that on mousover, affect a different element, target the child element, set display:block or opacity:100% depending on your previous choice. Set on-mouse-out to do the opposite.
You could also just give the child element an opacity of 0 and set it’s mousover state to opacity 100%. I usually don’t do this and prefer the first method, because there’s a way to automate it for all other thumbs, by giving the same class to all childs and checking the “limit to nested element” checkbox in the interaction panel.