This is Spanish for "what will be, will be." This is something you say when you are stuck in a hopelessly unchangeable situation, but have come to accept, or even embrace the unchangability of it all. This is similar to the phrase "it is what it is."
Doctor: I regret to say that your mother-in-law did not survive the surgery.
Guy: *attempting to suppress a smile* Oh well. Que sera sera.
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