Explaining Free Kriegsspiel to Various Types of Players
You've never played tabletop RPGs
It's an adventure game. The board is our shared imagination. One player acts as a referee, creating the world and characters to be found within. You handle your own characters, and imagine what you would do in their place. Most of the time, the referee will use his own knowledge and common sense to determine whether what you attempt to do works or not. Sometimes, he will ask the dice, or ask you to roll dice, to see what happens. What's special about this kind of game is that you have unlimited potential and possibilities within the confines of the setting, since there is no pre written story or assumption about how the game will play out. You might be heroes that end up saving the day, or villains terrorizing the populace, or something in between: you get to try whatever you want and deal with the consequences.
You've played Dungeons & Dragons 5e, or are mostly familiar with mainstream, "modern" school of games
Play world, not rules. Engaging with the system is no longer the priority. Instead, you almost only engage with the setting as we forego notions of game balance and system mastery in favour of a minimalist approach. You need to have absolute trust in the referee, as rule 0 is no longer "change the rules to suit play" but really "the referee IS the rules" - he'll call on them when he deems necessary, but they are here in case he needs them. Otherwise, things run as usual, except much faster paced and you get way more freedom of action since there are no prescriptive game elements outside of the in-fiction stuff.
You've played mostly Sorcerer, Apocalypse World, or Principled Freeform
Basically freeform, except you trust the referee to run the world, while he trusts you to run your characters. You get a traditional division of labour to maximize player-skill focused play and player agency with a focus on in-character decision making and the consequences that come from it. Rules are "under" the referee and players, and will probably be invisible - where negotiation happens is on matters of realism/logic.
You've played mostly OSR D&D, Classic Traveller and other Old Games from like, before the 80s
It's the same as usual but there's almost no player-facing rules whatsoever.