This refers to the pic(1) preprocessor to the troff(1) typesetter. Troff is a historical typesetting software with a long history in the Unix ecosystem. Having personally used some of the modern implementations (e.g. GNU troff or groff, heirloom troff) of troff for personal projects, it has been a pleasant experience.
One of the first “real” applications of Unix was in the AT&T documentation department. It not only reinforced the practicality of the Unixy “small tools linked by pipes” approach, it also encouraged the “little languages” approach to those tools, which brought us lex and yacc as people built things like eqn and pic to feed into troff.
To discover more, just google “unix little languages”.
