Abbreviation for the Latin word confer, meaning "compare." Cf. is a signal indicating that the cited source supports a different claim than the one just made, but that it is worthwhile to compare the two claims and assess the difference. Parenthetical explanations are encouraged to explain how a cf. source relates to the text.
It is sometimes said that while a cf. source supports a different claim than the author, it should be sufficiently similar so as to lend support to the author's claim. However, a better explanation might be that a cf. source simply offers a different yet non-contradictory claim; actual support should not be assumed. If the source gives a contradictory claim, a negative signal should be used.