Here's how it's used:
* Within a quote: "The student wrote, 'I like to eat apples, pears, and banannas [sic].'"
* This indicates that the word "banannas" is actually misspelled in the original student's writing, and the author is not responsible for the mistake.
* After a quote: "The sign read, 'Free coffe [sic]'. "
* Here, "sic" emphasizes that the misspelling of "coffee" is in the original sign, not the writer's error.
In essence, "sic" is a way of saying "this is exactly what the original said, even though it's wrong."