High level decision could mean anything that is not too specific, but is clear enough to understand what is the idea of the game. This is to avoid your team making the wrong idea of what you have in mind for the game, since "Stealth game" is a too broad term.
For example in your case of a Stealth game, these are some questions that would lead to High Level Design Decisions:
-Will the game end once the character is discovered? If not, can you fight those who discover you? Or do you have to hide again having no combat at all?
-Is the game divided in levels, or there is one big map with different points of interes?
-Can the player disable/incapacitate those who are looking for them? Has he no power over his enemies?
You can go on and on. But the idea is that you should focus on the most important aspects of your game, so it's clear to your team what the game is about and how does it feel.