What is the passé simple, and why/when is it used?

The passé simple, or passé historique, is a more formal or literary past tense in French. It conveys past, completed actions - not unlike the passé composé. You will often find it in literary texts, particularly older ones, and more formal journalistic texts. It is never used in spoken or informal French - if you use it in conversation, you might be mistaken for a time traveller! It's easy to spot, because it doesn't look like any of the other tenses you'll have learnt. For example, 'j'ai été' becomes 'je fus', and 'nous avons vendu' becomes 'nous vendîmes.'
It is unlikely that you will need to produce it in your A-Level exams, but it is useful to understand and recognise it, especially if you are studying literary texts or if you wish to pursue French to university level. If you're really keen to learn it, check out these conjugation tables at Lawless French (https://www.lawlessfrench.com/grammar/passe-simple-conjugations/) but do note that as always, there are many exceptions to the rule!