It's an acronym that's useful for stoy building. KISSBUTTS stands for;
Keep
It
Super
Simple
But
Understanding
The
Topic
&
Setting
It's a simple acronym that helps me with story building, and can be quite useful to keep you on track and not making a story have simple parts.
The "Keep It Super Simple" part is what you start off with for the basic plot and story. You want to make this huge world that's as complex as other stories? Baby steps, baby steps, you have to make a simple foundation that you can build upon.
Start with the basic characters/races/species/time period/tech level. Write a specific thing's information on a napkin for size, not a whole book, such as individual characters, races and general info. Keep the characters simple when we meet them, but have them grow as the story goes.
The "But Understanding The Topic & Setting" part means that you're following the foundation's plans properly without changing everything suddenly unless it works with the story. If your story is about aliens invading in 2005, it'd be a bit odd for someone to be carrying around a Windows 10 laptop. If your story's about actual history, you won't want goblins and orcs running around as actual goblins and orcs but you could use them for stuff like some political cartoons.
Whatever it is you're trying to make, you can always use KISSBUTTS to make it nicer for you and the readers.