Interpretation is quite different from explaining or 're-telling' what a text 'says' - that loses marks; interpretation always gains marks. When you interpret a text you look for the inferred 'deeper' layers of meaning that exist 'beneath' the surface details; when you 'retell' you merely explain the surface details - by putting them into your on words.
Interpretation also means working out how this deeper meaning has been 'shaped' and created by particular choices of language, style or structure. Think of this:
If a friend says to you, 'I'm hungry!' while you're eating a bag of crisps, you interpret the meaning easily as, 'Can I have a crisp?'. But when Shakespeare writes in his play, 'Macbeth', 'As two spent swimmers who cling together'... it's not so easy! You need to work out that he is not referring to swimming at all and that this must be a metaphorical use of language and so link what he does mean to the play in some useful way.
Similarly, when the poet, William Blake, writes, 'Tyger! Tyger! Burning bright!', you know he's not referring to animal cruelty. You need to work out what he does mean and state how this adds to and builds on or 'shapes' the overall meanings and themes of the poem - and why he decided to make his meaning in this unusually effective way.
A final example, when John Steinbeck, in his story 'Of Mice and Men', writes, 'The boss licked his pencil', you can guess that Steinbeck intends the meaning to have a deeper force than the mere surface meaning that the words literally say - in this case, you might interpret that the two ranch hands being interviewed, George Milton and Lennie Small, mean little more to the wealthy Boss than a mere 'lick of a pencil', i.e. that their humanity and value as human beings has been reduced virtually to zero in the cruel society of the 1930s US Great Depression.