Why Outlines?

Fonts can cause problems when sending artwork files to press. If you have used a typeface that your printing house does not have, the document or artwork will open and print incorrectly. This is why it is essential that you convert your fonts to outlines which means that the wording is no longer text - it has now become a graphic, and the text cannot be edited.

The top image is relying on font information to display correctly. If your printing house does not have the particular typeface used, the file will open but the font will be re-interpreted resulting in an incorrect reproduction.
The lower image, having been converted to outlines, behaves in the same way as a vector shape and does not rely on any typeface information.

More information on Converting Text To Outlines