to give a signal to begin; to tell someone that you will do what they want at the time they ask you

给一个开始的信号;吩咐一下，说一声，表示一下

If you need any help, just say the word.

需要我们帮忙你就说。

You only have to/Just say the word, and I'll come and help.

你只要说一声，我就会来帮忙。

I'm ready to start any time you say the word.

只要你说一声，我就准备开始。