An expression meaning that something is realized, understood, or experienced only after a certain state or condition is reached (not until ...).
親になって初めて、子育ての大変さが分かった。
It wasn't until I became a parent that I understood how hard raising children is.
実際に担当になって初めて、仕事の流れがよく分かった。
Only when I actually became responsible (for it) did I fully understand the workflow.
病気になって初めて健康のありがたさに気づいた。
I didn't appreciate my health until I got sick.
現場で作業をしてみになって初めて、設計上の問題点が見つかった。
It was only after actually working on the site that design problems were discovered.