When you paraphrase a sentence, you simply re-state it using different words, but still keep the same meaning.
For example:
"Romeo, Romeo, wherefore art thou Romeo?"
..can be paraphrased as:
"Romeo, Romeo, why did you have to be Romeo - why did you have to be who you are?
Usually, when someone paraphrases a sentence, they do it in a way that is more understandable to the person doing the paraphrasing.