Documented in Volume 1 of the UNIX Programmers Manual.
fgrep means "file grep"....
% fgrep -f wordlist textfile
the file wordlist contains lines which are each strings to be sought. The
file textfile contains text to be searched. When occurrences of a string
(line) in wordlist are found in textfile, the associated line from textfile
is listed on the standard output.
% spell filename | fgrep -v -x -f wordlist
will pass words not found in the system dictionary to fgrep, which will try
to match each such word with a LINE (-x) in file "wordlist" (-f). When
matches fail (-v) then list them to the standard output.
See also grep.