This feature is available in Ms-Word and Office 365 and it helps the user to set a shortcut of the content which user usually types every day so instead of typing, again and again, the same thing user can create a quick part of this content and use it whenever he/she needed it.
for example, if a user writes his/her signature
Thanks and Regards
at the end of every email then he/she can add his signature in the quick parts
This option is available under the insert menu i.e :
insert -> Quick Parts -> Save selection to the quick parts gallery
Following screen shows you how you can add the quick parts:
After creating quick part you can refer to it by just typing the name of the quick part
following the F3 key.