To be brief:

The gist of it is Norman Finkelstein spent alot of time and effort debunking https://en.wikipedia.org/wiki/From_Time_Immemorial by Joan Peters.

According to Peters, most people who call themselves Palestinians are not actually Palestinians, but instead descendants of recent immigrants from Saudi Arabia, Egypt, and Syria, who came to the land in waves of immigration starting in the 19th century and continuing through the period of the British Mandate. She argues that what is referred to as the 1948 Palestinian exodus was not ethnic cleansing, but actually a population exchange that resulted from the 1948 Arab–Israeli War.

Norman Finkelstein wrote that Peters' book was 'among the most spectacular frauds ever published on the Arab-Israeli conflict,'[17] arguing that its substance was based on extensive plagiarization of a work by Ernst Frankenstein published in the 1940s.[18] His 1984 review was based on his doctoral thesis, later expanded and published in Image and Reality of the Israel–Palestine Conflict. Finkelstein went into a close examination of all of Peters' notes and sources, and argued that her work persistently misrepresented or distorted the primary documents. His systematic critique of the book, attacking the two major pillars of Peters' thesis, which he regarded as a 'threadbare hoax' supported by the 'American intellectual establishment', had a major impact of later reviews of the book, especially those in Great Britain.

There is more to it obviously, with a surprise second-act appearance by Alan Dershowitz, but that is the broad strokes. It was actually quite a big deal and I remember the student protests when Finkelstein was denied tenure in 2007.