hail91. Please provide us with a complete sentence. You should also tell us where you found it and about the context in which it was being used. You may quote up to a total of 4 sentences of the original text if that makes the circumstances in which the phrase was used clearer.
Please note that I have changed the title of your post. I suggest that you read The Longer Guide to English Only to make sure that you understand how to ask a question here.
I'm so sorry.
this is the whole sentences from the text:
(Some banal love story about a nobleman and a lower middle class girl who can't marry and who then, out of sheer magnanimity,
renounce their love for each other)
It means that they acted completely unselfishly and with no thoughts of revenge against the system (or people?) who were the cause of their situation.
There is not much context in the sentence (or rather the near-sentence you gave) so it's not really clear why they were acting magnanimously.
Does this make sense to you? If not, perhaps you could quote the sentence before and the sentence after.