You can insert different types of "fields" into the text by choosing the ribbon command Insert | Field 
(in the group Text). The command can also be found under References (or Mailings) | Field.
Fields are placeholders that symbolize a specific piece of information. The "Print date" field, for example, stands for the current date; the "Page number" field stands for the current page number, etc. The available fields can even contain content from fields of a database that are needed for mail merges.
Whenever a document is opened or printed, all the fields it contains are updated automatically. Thus, if you reprint a document with the "Print date" field tomorrow, tomorrow's date will appear in the printout. Apart from that, you can manually update the fields of a document at any time by choosing the ribbon command References (or Mailings) | Update fields 
.
Fields can be formatted, deleted, copied, and moved like normal text, but their content cannot be edited directly.
The following pages provide detailed information on the use of fields.