"Thane of Fife" was a title used in medieval Scotland to refer to a nobleman granted land and authority over the region of Fife. The title was commonly bestowed by the King of Scotland to reward loyal service and to maintain control over the area.
Thane of Fife
Fife
Lady Macbeth
Macbeth is first 'Thane of Glamis' then he is bestowed the title 'Thane of Cawdor' then king of Scotland
He must watch out for Macduff
Duncan appoints him to that title, after it was stripped from the previous Thane. The former Thane of Cawdor was stripped of all titles and executed after joining a Norwegian invading force which was defeated at Fife. Macbeth at the time was at Forres, nowhere near Fife.
=Macduff The Thane of Fife=
Perhaps you are thinking of Lady Macbeth's line during her sleepwalking scene: "The thane of Fife had a wife--where is she now?"
"Beware Macduff, beware the Thane of Fife."
Macduff, the Thane of Fife, kills Macbeth.
Malcolm (Duncan's son) did not kill Macbeth, it was Macduff (Thane of Fife).
When Macbeth refers to Macduff he is talking about a man who holds the title Thane of Fife.