What Paper 1 Is
Paper 1 is the writing paper. You are given three prompts, each tied to a different prescribed theme, and you choose one to respond to. The response must be written in a specific text type, it must be aimed at a particular audience, and it must fulfil a clear purpose.
This is not a free-writing exercise, and it is not a content test. The examiner is not checking whether you know facts about the theme. They are assessing what you can do with English: how accurately and fluently you write, how well you develop ideas, and how confidently you handle the conventions of the text type you have been asked to produce.
Knowing a great deal about, say, the environment will not raise your mark if the writing itself is weak or if you ignore the text type.
Equally, impressive language that drifts away from the task loses marks on two criteria at once. Both things have to work together.