Nroff, or new roff, is a Unix OS text-formatting program that produces output for terminal windows and fixed-width printers. It is used to format manual pages and is an integral Unix help system component.
Nroff is a descendant of RUNOFF or roff.
Techopedia explains Nroff
Nroff script uses groff to emulate nroff commands and only accepts ASCII, Latin-1, UTF-8 and Cp1047 for output encoding. Nroff commands begin with a period, followed by two letters for command denotion. After the two-letter command, a number may be included to represent the number of spaces, lines or tabs.
A command, which must be located to the far left of a document, is placed directly above the line of applied text. Nroff only recognizes the period as the beginning of a command if the period is in this location.
Commands are easy to remember and use because they are often acronyms or command shortcuts, such as .bp (break point) and .in 3 (indent by 3 spaces). This type of document formatting is known as nroffing.