The difference between the installed fonts in the computer used to create a document (source computer) and the computer used to display or print the document (target computer). Computers come with a standard set of fonts. In order to use a different style of font, the target computer would have to have that matching font to render it accurately. This is the main reason people use the same Arial and Times New Roman fonts that come with Windows over and over again. They know that even old PCs will have those fonts.
Embedding the fonts from the source computer into the document itself solves this problem, which is the foundation of Adobe's Acrobat technology (PDF files). See PDF