Dictation is when someone listens to something that is being said and they write down exactly what that person says. For example, if I say to you “Today is July ninth and the weather is perfect.” you would write down, “Today is July ninth and the weather is perfect.” This was very common for secretaries to do many years ago before computers changed the need for this (though it still happens in court rooms).
This is also happens to be a wonderful listening and writing practice for you while you are learning English. It gets to you to focus and listen closely to what people are saying. It also gets you to focus on your writing/spelling to see if it is correct too.
Check out this website for hundreds of dictation exercises and good luck practicing your listening and writing skills!