They mean the exact same thing. "She's" is simply a contraction or shortened form of "She is." The apostrophe ' symbol in "She's" represents the letter "i" that is in the word "is."
Contractions are used a lot in English. Some common ones are:
don't = do not aren't = are not
won't = will not haven't = have not
can't = cannot it's = it is
And many, many more