Definition
To possess the necessary skills and knowledge to perform a task effectively.
心中有數
Don't worry, I know what I'm doing; I can handle this little task.
別擔心，我知道我在做什麼；我可以處理這個小任務。
Before starting any project, you need to make sure you know what you are doing.
在開始任何專案之前，你需要確保你知道自己在做什麼。
She is an experienced cook; of course, she knows what she is doing.
她是一位經驗豐富的廚師；當然，她知道自己在做什麼。