Here is image example and Dev Diary that it is from @Fovea
“One of the new features in CK3 is Tooltips in Tooltips, the aim of this is that relevant information can just be a mouse move away whenever you see blue highlighted text in game. Instead of making you need to dig through a wiki to find out what a Game Concept means you can just mouse over it and get an explanation!”
“This behaviour is nesting so you can get more and more information, so all the blue highlighted Game Concepts in this can also be highlighted to explain what a Ruler is and then what a Title itself is etc.”
"There are two different modes for Tooltips in Tooltips, timer lock and action lock with the former being the default. These modes determine what causes the Tooltips in Tooltips to lock in place and stay, either a timer which is configured in the settings menu or by clicking in the middle mouse button to manually lock them in place.
We have these two modes so that once you get more comfortable with the game you can turn the tooltips in tooltips into an opt in mode with the action lock for when you come across something you want to see instead of it being more present trying to aid you."