If your boss asks you to take dictation, then she’s going to say stuff out loud that she wants you to document in writing. When she begins dictation make sure you write down everything she says — no doodling.
If you’re a famous writer you might hire someone to take dictation so you can just speak and someone will write or type what you say. Dictation can describe the process of documenting spoken words, or the written words themselves. If you see an old movie with a man in a suit telling his secretary, "take this down," he's about to give dictation. A dictation can also be a command, like your dad's dictation that you mow the lawn.