Tooltip
Also said as Hover text, Nested tooltip, Keyword tip
A small box explaining the thing you are pointing at. A nested tooltip goes one further: words inside the box have tooltips of their own.
It lets a rich game explain itself at the moment of need, with no manual and no tutorial. It relies on hover, though, which doesn't exist on a touch screen or a pad, so those need another way in.
Say it
“Any game term in a tooltip gets its own tooltip, and it stays open while you move into it. On pad, the same tips open with a held button.”
Where you’ve seen it
Crusader Kings III is built on nested tooltips: hold your pointer over a term in a tip and it opens another tip, as deep as you care to go, so a very complicated game can explain itself in place. Slay the Spire explains keywords like Vulnerable right beside the card.
Not to be confused with tutorial
A tutorial teaches you something when the game decides. A tooltip answers a question when you ask it.