What is the past historic tense?

The past historic is used in the same way that the passé composé (the perfect) is: to describe actions completed in the past.
You should never use the passé composé when speaking. It is incredibly rare and is only used in very very formal situations: you never need to do this at A-level or even at university level.
In terms of writing, the passé composé is used in formal writing. It is usually used in literature, so this is where you may have come across it. You will not need to have to write this tense at A-level, but you will at university level. It is good to be able to recognise this tense and understand why it is being used.
The slight difference between the passé composé and the past historic (aside from how formal the latter is) is that actions in the past historic typically have no connection to the present, whereas actions in the passé composé may have more of a link to the present.
For A-level, focus on using the passé composé (and when this is used compared to the imperfect tense), but be able to recognise the past historic.