font-lock-add-keywords is a compiled Lisp function in `font-lock.el'.

(font-lock-add-keywords MODE KEYWORDS &optional HOW)

Add highlighting KEYWORDS for MODE.

MODE should be a symbol, the major mode command name, such as `c-mode'
or nil. If nil, highlighting keywords are added for the current buffer.
KEYWORDS should be a list; see the variable `font-lock-keywords'.
By default they are added at the beginning of the current highlighting list.
If optional argument HOW is `set', they are used to replace the current
highlighting list. If HOW is any other non-nil value, they are added at the
end of the current highlighting list.

For example:

(font-lock-add-keywords 'c-mode
'(("\\<\\(FIXME\\):" 1 'font-lock-warning-face prepend)
("\\<\\(and\\|or\\|not\\)\\>" . 'font-lock-keyword-face)))

adds two fontification patterns for C mode, to fontify `FIXME:' words, even in
comments, and to fontify `and', `or' and `not' words as keywords.

The above procedure will only add the keywords for C mode, not
for modes derived from C mode. To add them for derived modes too,
pass nil for MODE and add the call to c-mode-hook.

For example:

(add-hook 'c-mode-hook
(lambda ()
(font-lock-add-keywords nil
'(("\\<\\(FIXME\\):" 1 'font-lock-warning-face prepend)
("\\<\\(and\\|or\\|not\\)\\>" .

The above procedure may fail to add keywords to derived modes if
some involved major mode does not follow the standard conventions.
File a bug report if this happens, so the major mode can be corrected.

Note that some modes have specialized support for additional patterns, e.g.,
see the variables `c-font-lock-extra-types', `c++-font-lock-extra-types',
`objc-font-lock-extra-types' and `java-font-lock-extra-types'.