If you have particular qualities at your command, you are able to use them effectively.

As a writer, she has both style and humour at her command. 作为一名作家，她对文体和幽默都能运用自如。

If someone says that they are at your command, that person is willing to do what you ask.

"I can't reach my zip - could you unfasten it, please?" "I'm at your command!" “我够不着我的拉链——请你帮我拉开好吗？”“遵命！”