I believe the equivalent term used in English (exhibited in the new translation) is "natural", short for "natural expansion", which refers to the obvious location where the player should build their first expansion. It sounds like the term used in Korean for this concept literally means "front yard" rather than matching the English term.
Not the parent commenter but not always. 9 pool just means you build a spawning pool at your main, for instance. This worker-prefix building build-order naming system also breaks down once people start referencing builds like 2 rax academy, 3 hatch muta, etc.
I think strictly "9 pool" means you build the pool when you have 9 supply. However, before you build a spawning pool, the only thing you can build that consumes supply is workers.
