Picture a building as a pile of floors. One of them sits on the earth, and that is the one you walk into from the street: no to climb, nothing under you but the ground. Everything else in the building is built on top of it. English keeps the same picture when it talks about time, not space. To be in on the ground floor is to be there while the thing is still only one floor high, before anything has been built above it.
可以想成一棟樓是一層一層疊起來的，其中有一層直接踩在地面上。那一層就是你從馬路一步走進去、不必爬樓梯的那一層。它下面沒有別層，整棟樓都蓋在它上面。英文談時間的時候也借用同一個畫面：一件事還只有最底下那一層、上面什麼都還沒蓋起來，你就已經在裡面了。