The most important things you need to do when answering this question are:
to show you understand the poem,
and
to explore some of the subtleties of how it is written.
The examiners at AQA, for example, are looking for ‘a convincing critical analysis and exploration’ in the top-mark answers.
Within that broad definition, there are two important ‘Assessment Objectives’ -
response to the poem
critical analysis of how meaning is created.
At a basic level, the exam question will require you to show that you understand clearly what the poem is saying - ‘what it’s about’.
To reach the higher marks, though, you will have to show a detailed and interesting appreciation of what makes the poem special.
Poems by their very nature are ‘special language’. They don’t exist just to give information (contrast a poem, for example, with an instruction manual). Poems normally interact creatively with our imagination and our feelings, creating meanings which are often fascinating in their complexity.
At GCSE level, you won’t be expected to write about all the possible complexities of a poem you are reading for the first time on the exam paper.
But you will be expected to explore some of the qualities which make the poem special, different from everyday language, and different too, perhaps, from other poems (comparison with a second ‘unseen’ poem is an important part of the question in the AQA GCSE).
Here are some main areas on which you should focus in your answer: