A hard return is an actual symbol inserted into the text. The hard-return symbol is usually invisible, but most word processors support a mode that lets you see them. Whenever you press the Return or Enter key while editing a document, the word processor inserts a hard return.
Hard returns cause the word processor to start a new line regardless of how margins are set. Therefore, if a document contains hard returns, the lines will end at the same place even if you change the margins. In contrast, soft returns, inserted by the program rather than the user, depend on how the margins are set.