Embedding a font means that the font is included in the document. If it helps, think of "embedded" as "in the same bed".
Word documents are a good example. If you type something in, say, Reminder Pro
(or some other obscure font), and send it to someone who (most likely) doesn't have that font, Word will have to substitute it with something else. But some e-readers may not be that smart - especially the cheap ones - and the text may show up as blank; selectable and searchable text, but blank.
If you have Adobe Reader installed, open the PDF and go to File - Properties - Fonts. Fonts are usually embedded as subsets
, meaning that they only contain the characters that were used in the document instead of the whole font. This can shave off a few dozen KB or so, depending on the document's content and the number of glyphs in the actual font.
Next time please use the search function. Googling is also a very good idea.