The audience for a piece of writing is its reader. This may seem too obvious to be worth mentioning. But no. This is because each year exam boards tell schools that very many students fail to take proper account of their audience and lose many marks as a result.
When analysing a play for example, you should consider both the original audience (for example, Shakespeare's audience of the early seventeenth century) and a modern audience, taking account of the different interpretations each would make of the play. The same applies to all texts.
In writing of your own, your teacher is looking for is...
Evidence that you have chosen an appropriate style to suit the audience in order that the purpose of the writing might be fulfilled.
So always consider the requirements of your audience and choose an appropriate style (such as tone of voice, level of formality, vocabulary, etc.) to suit both them and the purpose of the writing. After all, without a reader, the text has no value.
Importantly in writing to argue or persuade... don't make the audience seem foolish and if you write to inform... be balanced and let your readers make up their mind on the issue if you are writing to explain... be clear and don't patronise or be too simplistic and if you are writing to describe... aim to etch vivid sensory images (what you can see, hear, smell, taste, touch...) onto your reader's mind by using vivid similes and metaphors!